Complex objective lens, optical head, optical information apparatus, computer, optical disk player, car navigation system, optical disk recorder, and optical disk server

ABSTRACT

A complex objective lens composed of a hologram and an objective lens, capable of realizing stable and high-precision compatible reproducing/recording of a BD with a base thickness of about 0.1 mm for a blue light beam (wavelength λ 1 ) and a DVD with a base thickness of about 0.6 mm for a red light beam (wavelength λ 2 ). In an inner circumferential portion of the hologram, a grating is formed, which has a cross-sectional shape including as one period a step of heights in the order of 0 time, twice, once, and three times a unit level difference that gives a difference in optical path of about one wavelength with respect to a blue light beam, from an outer peripheral side to an optical axis side. The hologram transmits a blue light beam as 0th-order diffracted light without diffracting it, and disperses a red light beam passing through an inner circumferential portion as +1st-order diffracted light and allows it to be condensed by an objective lens. Because of this, the focal length of the red light beam becomes longer than that of the blue light beam, whereby a working distance is enlarged.

This application is a Continuation of application Ser. No. 10/453,073,filed Jun. 2, 2003 now U.S. Pat. No. 7,245,407, which application isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a complex objective lens in which anobjective lens is combined with a hologram that is a diffractionelement; an optical head for condensing light beams having a pluralityof wavelengths onto an optical disk via the complex objective lens torecord, reproduce, or delete information; an optical informationapparatus in which the optical head is mounted; and a computer, anoptical disk player, a car navigation system, an optical disk recorderand an optical disk server to which the optical information apparatus isapplied.

2. Description of the Related Art

An optical memory technique using an optical disk having a pit-shapedpattern as a storage medium with a high density and a large capacity isbeing put to practical use while extending the range of uses to adigital audio disk, a video disk, a document file disk, and a data file.A function of recording/reproducing information with respect to anoptical disk satisfactorily with high reliability, using a minutelycondensed light beam, is roughly classified into a condensing functionof forming a minute spot of a diffraction limit, focal point control(focus servo) and tracking control of an optical system, and detectionof a pit signal information signal).

Recently, because of the advancement of an optical system designtechnique and a decrease in wavelength of a semiconductor laser that isa light source, a high-density optical disk having a storage capacitylarger than that of the prior art is being developed. As an approach tolarger densities, an increase in a numerical aperture (NA) on an opticaldisk side of a condensing optical system that condenses a light beamminutely onto an optical disk is being studied. In this case, there is aproblem that the amount of aberration caused by the inclination (i.e.,tilt) of an optical axis is increased. When a NA is increased, theamount of aberration occurring with respect to a tilt also is increased.In order to prevent this, the thickness (base thickness) of a substrateof an optical disk should be made thinner.

A compact disk (CD) that may be a first generation optical disk usesinfrared light (wavelength λ3: 780 nm to 820 nm) and an objective lenswith a NA of 0.45, and has a base thickness of 1.2 mm. A DVD that is asecond generation optical disk uses red light (wavelength λ2: 630 nm to680 nm; standard wavelength: 660 nm) and an objective lens with a NA of0.6, and has a base thickness of 0.6 mm. Furthermore, a third generationoptical disk hereinafter, referred to as a BD (Blue-ray Disk) uses bluelight (wavelength λ1: 390 nm to 415 nm; standard wavelength: 405 nm) andan objective lens with a NA of 0.85, and has a base thickness of 0.1 mm.In the present specification, the base thickness refers to a thicknessfrom a surface of an optical disk (or an information medium) upon whicha light beam is incident to an information recording surface.

Thus, the base thickness of an optical disk is decreased with anincrease in density. In terms of economical points and a space occupiedby an apparatus, there is a demand for an optical information apparatuscapable of recording/reproducing information with respect to opticaldisks having different base thicknesses and recording densities. Inorder to achieve this, an optical head is required that is provided witha condensing optical system capable of condensing a light beam to adiffraction limit onto optical disks having different base thicknesses.

Furthermore, in the case where information is recorded/reproduced withrespect to an optical disk having a thick base material, it is necessaryto condense a light beam onto a recording surface that is positioned ona deeper side of the disk surface. Therefore, a focal length needs to bemade larger.

JP 7(1995)-98431 A discloses a configuration intended to realize anoptical head that records/reproduces information with respect to opticaldisks having different base thicknesses. This configuration will bedescribed as a first conventional example with reference to FIGS. 25Aand 25B.

In FIGS. 25A and 25B, reference numerals 40 and 41 denote an objectivelens and a hologram, respectively. The hologram 41 is provided with aconcentric grating pattern on a substrate transparent to an incidentlight beam 44.

The objective lens 40 has an numerical aperture NA of 0.6 or more, andas shown in FIG. 25A, is designed so as to allow 0th-order diffractedlight 42 that passes through the hologram 41 without being diffracted toform a condensed spot of a diffraction limit on an optical disk 10, forexample, having a base thickness (t2) of 0.6 mm. Furthermore, FIG. 25Bshows that a condensed light spot of a diffraction limit can be formedon an optical disk 11 having a larger base thickness (t1) (i.e., 1.2mm). In FIG. 25B, +1st-order diffracted light 43 diffracted by thehologram 41 is condensed onto the optical disk 11 by the objective lens40. Herein, the +1st-order diffracted light 43 is subjected toaberration correction so as to be condensed to a diffraction limitthrough a substrate with a thickness t1.

Thus, by combining the hologram 41 that diffracts incident light withthe objective lens 40, a 2-focal point lens is realized, which iscapable of forming a condensed light spot that is condensed to adiffraction limit on the optical disks 10, 11 having different basethicknesses (t1 and t2), using diffracted light of different orders.Furthermore, it also is disclosed that, conversely to the above, thehologram 41 is designed so as to have a convex lens action, and0th-order diffracted light is used for the optical disk 11 with the basethickness t1, and +1st-order diffracted light is used with respect tothe optical disk 10 with the base thickness t2, whereby a fluctuation ina focal point position can be reduced with respect to a wavelengthfluctuation during recording/reproducing of information with respect tothe optical disk having the base thickness t2.

There also is a disclosure of a configuration intended for compatiblereproducing of information with respect to optical disks havingdifferent kinds, using light beams having a plurality of wavelengths. Asa second conventional example, a configuration in which a wavelengthselection phase plate is combined with an objective lens is disclosed byJP 10(1998)-334504 A and Session We-C-05 of ISOM2001 (page 30 of thepreprints). The configuration disclosed by Session We-C-05 of ISOM2001(Page 30 of the preprints) will be described with reference to FIGS. 26,27A and 27B.

FIG. 26 is a cross-sectional view showing a schematic configuration ofan optical head as the second conventional example. In FIG. 26, parallellight output from a blue light optical system 51 having a blue lightsource (not shown) with a wavelength λ1 of 405 nm passes through a beamsplitter 161 and a wavelength selection phase plate 205 and is condensedonto an information recording surface of an optical disk 9 (thirdgeneration optical disk: BD) with a base thickness of 0.1 mm by anobjective lens 50. The light reflected from the optical disk 9 follows areverse path and is detected by a detector (not shown) of the blue lightoptical system 51. On the other hand, divergent light output from a redlight optical system 52 having a red light source (not shown) with awavelength λ2 of 660 nm is reflected by the beam splitter 161, passesthrough the wavelength selection phase plate 205, and is condensed ontothe information recording surface of an optical disk 10 (secondgeneration optical disk: DVD) with a base thickness of 0.6 mm by theobjective lens 50. Light reflected from the optical disk 10 follows areverse path and is detected by a detector (not shown) of the red lightoptical system 52.

The objective lens 50 is designed so as to allow parallel light to passthrough the optical disk 9 with the base thickness of 0.1 mm to becondensed. Therefore, for recording/reproducing of information withrespect to the DVD with the base thickness of 0.6 mm, sphericalaberration is caused by the difference in base thickness. In order tocorrect the spherical aberration, a light beam output from the red lightoptical system 52 is formed into dispersed light, and the wavelengthselection phase plate 205 is used. When dispersed light is incident uponthe objective lens 50, new spherical aberration occurs. Therefore, thespherical aberration caused by the difference in base thickness iscancelled by the new spherical aberration, and a wavefront is correctedby the wavelength selection phase plate 205.

FIGS. 27A and 27B respectively are a plan view and a cross-sectionalview of the wavelength selection phase plate 205 in FIG. 26. Thewavelength selection phase plate 205 is configured with a leveldifference 205 a between heights h and 3 h, assuming that the refractiveindex at a wavelength λ1 is n1, and h=λ1/(n1−1). The optical pathdifference caused by the level difference of the height h is a usewavelength λ1, which corresponds to a phase difference 2π. This case isthe same as the phase difference of 0. Therefore, the level differenceof the height h does not influence the phase distribution of a lightbeam with a wavelength λ1, and does not influence recording andreproducing of information with respect to the optical disk 9 (FIG. 26).On the other hand, assuming that the refractive index of the wavelengthselection phase plate 205 at a wavelength λ2 is n2, h ×(n2−1)/λ2≈0.6,i.e., an optical path difference that is not an integral multiple of awavelength occurs. The above-mentioned aberration correction isperformed by using a phase difference caused by the optical pathdifference.

Furthermore, as a third conventional example, JP 11(1999)-296890 A andthe like disclose a configuration in which a plurality of objectivelenses are switched mechanically.

Furthermore, as a fourth conventional example, JP 11(1999)-339307 Adiscloses a configuration in which a mirror having a reflective surfacewith different radii of curvature also functions as a rising mirror(changing the direction of light from a horizontal direction to avertical direction so that the light is incident upon an optical disk)that bends an optical axis.

As a fifth conventional example, JP 2000-81566 A discloses aconfiguration in which a refraction type objective lens is combined witha hologram in the same way as in the first conventional example, and thedifference in base thickness is corrected by using chromatic aberrationcaused by diffracted light of the same order as that of light having adifferent wavelength

As a sixth conventional example, as shown in FIG. 28, a configuration inwhich a refraction type objective lens 281 is combined with a hologram282 having a diffraction surface and a refraction surface is describedin “BD/DVD/CD Compatible Optical Pickup Technique” by Sumito Nishioka(Extended Abstracts (50th Spring Meeting); The Japan Society of AppliedPhysics, 27p-ZW-10 (Kanagawa University, March 2003)) (published afterfiling of the priority application of the present application). In thesixth conventional example, the hologram 282 is allowed to generate+2nd-order diffracted light with respect to a blue light beam and+1st-order diffracted light with respect to a red light beam, wherebychromatic aberration correction is performed. Furthermore, dispersedlight is allowed to be incident upon the hologram 282 and the objectivelens 281 with respect to a blue light beam, and converged light isallowed to be incident upon them with respect to a red light beam,whereby spherical aberration caused by a difference in base thickness iscorrected.

The above-mentioned first conventional example proposes at least thefollowing three technical ideas. First, the compatibility of opticaldisks having different base thicknesses is realized by using thediffraction of a hologram. Second, the design of an inner/outerperiphery is changed to form condensed light spots having different NAs.Third, a focal point position of a condensed light spot is changed withrespect to optical disks having different base thicknesses by using thediffraction of a hologram. These technical ideas do not limit thewavelength of light to be emitted by a light source.

Herein, a DVD that is the second generation optical disk includes atwo-layer disk having two recording surfaces. The recording surface(first recording surface) on a side closer to an objective lens needs toallow light to pass through to a surface away from the objective lens,so that its reflectivity is set at about 30%. However, this reflectivityis guaranteed only with respect to red light, and is not guaranteed atthe other wavelengths. Therefore, in order to exactly reproduceinformation from a DVD, it is required to use red (wavelength λ2=630 nmto 680 nm) light. Furthermore, in recording/reproducing of informationwith respect to a BD that is the third-generation optical disk, it isrequired to use blue (wavelength λ1=390 nm to 415 nm) light so as todecrease the diameter of a condensed light spot sufficiently. Thus, thefirst conventional example does not disclose the configuration in whicha light use efficiency is enhanced further when different kinds ofoptical disks are made compatible using, in particular, red light andblue light.

Furthermore, the first conventional example discloses an example inwhich a hologram is formed in a convex lens type, and +1st-orderdiffracted light is used, whereby the movement of a focal point positiondue to a change in wavelength is reduced with respect to one kind ofoptical disk. However, the first conventional example does not disclosea scheme of reducing simultaneously the movement of a focal pointposition caused by a change in wavelength with respect to each of atleast two kinds of optical disks.

The second conventional example uses a wavelength selection phase plateas a compatible element. When information is recorded/reproduced withrespect to a disk having a large base thickness, a recording surface ispositioned away from an objective lens by the base thickness. Therefore,it is required to increase a focal length. The focal length can beincreased by providing the compatible element with a lens power.However, the wavelength selection phase plate does not have a lenspower. Furthermore, as in the second conventional example, when it isattempted to realize all the lens powers with respect to dispersed redlight, a large aberration occurs while an objective lens is moved (e.g.,follows a track), resulting in degradation of recording/reproducingcharacteristics.

In the third conventional example, since objective lenses are switched,it is required to use a plurality of objective lenses, which increasesthe number of parts, and makes it difficult to miniaturize an opticalhead. Furthermore, the requirement of a switching mechanism also makesit difficult to miniaturize an apparatus.

In the fourth conventional example, an objective lens is drivenindependently of a mirror (see FIGS. 4 to 6 in JP 11(1999)-339307 A).However, a light beam is converted from parallel light by a mirrorhaving the above-mentioned radius of curvature. Therefore, when theobjective lens is moved by track control or the like, the relativeposition of the objective lens with respect to an incident lightwavefront is changed, aberration occurs, and condensing characteristicsare degraded. Furthermore, the reflective surface of a mirror iscomposed of a surface having a radius of curvature i.e., a sphericalsurface); however, the spherical surface is not sufficient forcorrecting the difference in base thickness and the difference inwavelength, and high-order aberration (5th-order or higher) cannot bereduced sufficiently.

When the fifth conventional example is applied directly to a red lightbeam and a blue light beam, diffraction efficiencies of the same ordercannot be enhanced simultaneously because of a large difference inwavelength, and consequently, the use efficiency of light is decreased.

In the sixth conventional example, dispersed light is allowed to beincident upon a hologram and an objective lens with respect to a bluelight beam and converged light to be incident upon them with respect toa red light beam. Therefore, under the condition of focusing (i.e.,under the condition that a condensed light spot of a diffraction limitis formed on an information recording surface of an optical disk), alight beam reflected to be returned from an optical disk also becomesdifferent in a parallel degree between a blue light beam and a red lightbeam, and a photodetector for detecting a servo signal cannot be sharedbetween a blue light beam and a red light beam. More specifically, atleast two photodetectors are required, which increase the number ofparts, leading to an increase in cost.

SUMMARY OF THE INVENTION

Therefore, with the foregoing in mind, it is an object of the presentinvention to provide a complex objective lens having a high light useefficiency, that realizes compatible reproducing and compatiblerecording between an optical disk designed for a red light beam with awavelength λ2 (typically, about 660 nm) at a base thickness 0.6 mm andan optical disk designed for a blue light beam at a wavelength λ1(typically, about 405 nm) at a base thickness of 0.1 mm.

It is another object of the present invention to provide an opticalinformation apparatus capable of handling a plurality of optical diskshaving different recording densities with a single optical head bymounting an optical head using the above-mentioned complex objectivelens.

It is still another object of the present invention to provide acomputer, an optical player, a car navigation system, an optical diskrecorder, and an optical disk server capable of recording/reproducinginformation stably by selection of different kinds of optical disks inaccordance with the use, by including the above-mentioned opticalinformation apparatus.

In order to achieve the above-mentioned object, a first complexobjective lens according to the present invention includes a hologramand a refraction type lens, wherein the hologram has a grating with astepped cross-section, a level difference of the stepped cross-sectionis an integral multiple of a unit level difference d1, the unit leveldifference d1 gives a difference in optical path length of about onewavelength to a first light beam having a wavelength λ1 in a range of390 nm to 415 nm, and one period of the grating is composed of a step ofheights in an order of 0 time, twice, once, and three times the unitlevel difference d1 from an outer peripheral side to an optical axisside of the hologram.

In the first complex objective lens, a ratio of widths of the leveldifference of the stepped cross-section of the grating is 2:3:3:2corresponding to the heights in the order of 0 time, twice, once, andthree times the unit level difference d1.

Furthermore, in the first complex objective lens, the grating is formedonly in an inner circumferential portion of the hologram.

Furthermore, the first complex objective lens condenses 0th-orderdiffracted light of the first light beam through a base with a thicknesst1 and condenses 1st-order diffracted light of a second light beamhaving a wavelength λ2 in a range of 630 nm to 680 nm through a basewith a thickness t2 larger than the thickness t1.

In order to achieve the above-mentioned object, a second complexobjective lens according to the present invention includes a hologramand a refraction type lens, wherein the hologram has a grating with astepped cross-section formed in at least an inner circumferentialportion, a level difference of the stepped cross-section is an integralmultiple of a unit level difference d2, the unit level difference d2gives a difference in optical path length of about 1.25 wavelengths to afirst light beam having a wavelength λ1 in a range of 390 nm to 415 nm,and one period of the grating is composed of a step of heights in anorder of 0 time, once, twice, and three times the unit level differenced2 from an outer peripheral side to an optical axis side of thehologram.

In the second complex objective lens, a ratio of widths of the leveldifference of the stepped cross-section of the grating is 1:1:1:1corresponding to the heights in the order of 0 time, once, twice, andthree times the unit level difference d2.

Furthermore, in the second complex objective lens, the hologram has agrating with a stepped cross-section formed in an outer peripheralportion, a level difference of the stepped cross-section of the gratingformed in the outer peripheral portion is an integral multiple of a unitlevel difference d3, the unit level difference d3 gives a difference inoptical path length of about 0.25 wavelengths to the first light beam,and one period of the grating formed in the outer peripheral portion iscomposed of a step of heights in an order of 0 time, once, twice andthree times the unit level difference d3 from an outer peripheral sideto an optical axis side of the hologram.

Furthermore, the second complex objective lens condenses +1st-orderdiffracted light of the first light beam through a base with a thicknesst1 and condenses −1st-order diffracted light of a second light beamhaving a wavelength λ2 in a range of 630 nm to 680 nm passing through agrating formed in an inner circumferential portion of the hologramthrough a base with a thickness t2 larger than the thickness t1.

In order to achieve the above-mentioned object, a third complexobjective lens according to the present invention includes a hologramand a refraction type lens, wherein the hologram has a grating with asawtooth cross-section formed in at least an inner circumferentialportion, and a depth h1 of the sawtooth cross-section gives a differencein optical path length of about 2 wavelengths to a first light beamhaving a wavelength λ1 in a range of 390 nm to 415 nm to allow the firstlight beam to generate +2nd-order diffracted light most strongly, andallows a second light beam having a wavelength λ2 in a range of 630 nmto 680 nm to generate +1st-order diffracted light most strongly.

In order to achieve the above-mentioned object, a fourth complexobjective lens according to the present invention includes a hologramand a refraction type lens, wherein the hologram has a grating with asawtooth cross-section formed in at least an inner circumferentialportion, and a depth h2 of the sawtooth cross-section gives a differencein optical path length of about one wavelength to a second light beamhaving a wavelength λ2 in a range of 630 nm to 680 nm to allow thesecond light beam to generate +1st-order diffracted light most strongly,and allows a first light beam having a wavelength λ1 in a range of 390nm to 415 nm to generate +2nd-order diffracted light most strongly.

In order to achieve the above-mentioned object, a fifth complexobjective lens according to the present invention includes a hologramand a refraction type lens, wherein the hologram has a grating with asawtooth cross-section formed in at least an inner circumferentialportion, and a depth h4 of the sawtooth cross-section gives a differencein optical path length larger than 1.7 wavelengths and smaller than 2wavelengths to a first light beam having a wavelength λ1 in a range of390 nm to 415 nm to allow the first light beam to generate +2nd-orderdiffracted light most strongly, and allows a second light beam having awavelength λ2 in a range of 630 nm to 680 nm to generate +1st-orderdiffracted light most strongly.

In the fifth complex objective lens, it is preferable that the depth h4of the sawtooth cross-section gives a difference in optical path lengthof 1.9 wavelengths to the first light beam.

In order to achieve the above-mentioned object, a sixth complexobjective lens according to the present invention includes a hologramand a refraction type lens, wherein the hologram allows a first lightbeam having a wavelength λ1 in a range of 390 nm to 415 nm to generate+2nd-order diffracted light most strongly and allows a second light beamhaving a wavelength λ2 in a range of 630 nm to 680 nm to generate+1st-order diffracted light most strongly, and the refraction type lenscondenses the +2nd-order diffracted light of the first light beam viathe hologram through a base with a thickness t1, and condenses+1st-order diffracted light of the second light beam via an innercircumferential portion of the hologram through a base with a thicknesst2 larger than the thickness t1.

In the third to sixth complex objective lenses, the hologram has agrating with a sawtooth cross-section formed in an outer peripheralportion, and a depth h3 of the sawtooth cross-section of the gratingformed in the outer peripheral portion gives a difference in opticalpath length of about one wavelength to the first light beam to allow thefirst light beam to generate +1st-order diffracted light most strongly,and allows the second light beam to generate +1st-order diffracted lightmost strongly.

In the second to sixth complex objective lenses, the hologram isconfigured so as to have a function as a convex lens in order to reducea change in a focal length with respect to a change in the wavelength λ1in a case of condensing the first light beam through a base with athickness t1.

In the first to sixth complex objective lenses, in order to place afocal point position on an optical disk side away from the complexobjective lens, the hologram is configured so as to have a largerfunction as a convex lens compared with the case of condensing thesecond light beam passing through an inner circumferential portion ofthe hologram through the base with the thickness t2, in a case ofcondensing the first light beam through the base with the thickness t1,or the hologram is configured so as to have a smaller function as aconvex lens compared with a case of condensing the first light beamthrough the base with the thickness t1, in a case of condensing thesecond light beam passing through the inner circumferential portion ofthe hologram through the base with the thickness t2. Because of this,the focal point position on the optical disk side can be placed awayfrom the complex objective lens, i.e., a working distance can beenlarged.

In the third to sixth complex objective lenses, a cross-sectional shapeof the grating constituting the hologram is a sawtooth shape with a baseforming the hologram having a slope on an outer peripheral side.

In the first to sixth complex objective lenses, it is preferable thatthe hologram and the refraction type lens are fixed integrally.

Alternatively, in the first to sixth complex objective lenses, it ispreferable that a refractive surface of the refraction type lens on anopposite side of a condensed spot is an aspherical surface. In thiscase, it is preferable that the hologram is formed integrally on theaspherical surface of the refraction type lens.

Alternatively, in the first to sixth complex objective lenses, thehologram is formed integrally on a surface of the refraction type lens.

In the first to sixth complex objective lenses, assuming that anumerical aperture at which the first light beam is condensed throughthe base with the thickness t1 is NAb, and a numerical aperture at whichthe second light beam is condensed through the base with the thicknesst2 is NAr, NAb>NAr is satisfied.

In order to achieve the above-mentioned object, an optical headapparatus according to the present invention includes: a first laserlight source for emitting a first light beam having a wavelength λ1 in arange of 390 nm to 415 nm; a second laser light source for emitting asecond light beam having a wavelength λ2 in a range of 630 nm to 680 nm;one of the first to sixth complex objective lenses for receiving thefirst light beam emitted from the first laser light source to condenseit onto a recording surface of a first optical disk through a base witha thickness t1, and receiving the second light beam emitted from thesecond laser light source to condense it onto a recording surface of asecond optical disk through a base with a thickness t2 larger than thethickness ti; and a photodetector for receiving the first and secondlight beams reflected respectively from the recording surfaces of thefirst and second optical disks to output an electric signal inaccordance with light amounts of the first and second light beams.

It is preferable that the optical head apparatus according to thepresent invention includes a collimator lens that collimates the firstand second light beams respectively emitted from the first and secondlaser light sources, wherein when the second light beam is condensedonto the recording surface of the second optical disk, the collimatorlens is placed closer to the second laser light source side to convertthe second light beam to divergent light so as to allow it to beincident upon the complex objective lens, whereby a focal point positionon the second optical disk side is placed away from the complexobjective lens.

In the optical head apparatus according to the present invention, thefirst and second laser light sources are placed so that both lightingpoints thereof have an image-forming relationship with respect to focalpoint positions of the complex objective lens on the first and secondoptical disk sides, and the photodetector is provided so as to be sharedby the first and second light beams respectively reflected from therecording surfaces of the first and second optical disks and receivesthe first and second light beams to detect a servo signal.

In order to achieve the above-mentioned object, an optical informationapparatus according to the present invention includes: the optical headapparatus according to the present invention; a motor for rotating thefirst and second optical disks; and an electric circuit for receiving asignal obtained from the optical head apparatus and driving the motor,the complex objective lens, and the first and second laser light sourcesbased on the signal.

In the optical information apparatus according to the present invention,the optical head apparatus includes a collimator lens that collimatesthe first and second light beams respectively emitted from the first andsecond laser light sources, and when the second optical disk having abase with a thickness t2 of 0.6 mm is mounted, the optical informationapparatus according to the present invention moves the collimator lensto the second laser light source side.

In order to achieve the above-mentioned object, a computer according tothe present invention includes: the optical information apparatusaccording to the present invention; an input apparatus for inputtinginformation; an arithmetic unit for performing an arithmetic operationbased on information input from the input apparatus and informationreproduced from the optical information apparatus; and an outputapparatus for displaying or outputting the information input from theinput apparatus, the information reproduced from the optical informationapparatus, and a result of the arithmetic operation by the arithmeticunit.

In order to achieve the above-mentioned object, an optical disk playeraccording to the present invention includes: the optical informationapparatus according to the present invention; and a decoder forconverting an information signal obtained from the optical informationapparatus to an image signal.

In order to achieve the above-mentioned object, a car navigation systemaccording to the present invention includes: the optical informationapparatus according to the present invention; and a decoder forconverting an information signal obtained from the optical informationapparatus to an image signal.

In order to achieve the above-mentioned object, an optical disk recorderaccording to the present invention includes: the optical informationapparatus according to the present invention; and an encoder forconverting an image signal to an information signal to be recorded inthe optical information apparatus.

In order to achieve the above-mentioned object, an optical disk serveraccording to the present invention includes: the optical informationapparatus according to the present invention; and an input/outputterminal for recording an information signal input from outside in theoptical information apparatus and outputting an information signalreproduced from the optical information apparatus to outside.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing one exemplary configuration ofan optical head according to Embodiment 1 of the present invention.

FIG. 2 is a cross-sectional view showing a specific example of a complexobjective lens composed of a hologram 13 and an objective lens 14 inFIG. 1.

FIG. 3A is a plan view showing a configuration of the hologram 131 inFIG. 2.

FIG. 3B is a cross-sectional view showing a configuration of thehologram 131 in FIG. 2.

FIG. 4A is a cross-sectional view showing a stepped shape in one period(p1) of a grating formed in an inner circumferential portion 131C of thehologram 131 shown in FIG. 3A.

FIG. 4B shows a phase modulation amount with respect to a red light beam62 (wavelength λ2) corresponding to FIG. 4A.

FIG. 5 is a cross-sectional view showing a specific example of a complexobjective lens composed of the hologram 13 and the objective lens 14shown in FIG. 1 in Embodiment 2 of the present invention.

FIG. 6A is a plan view showing a configuration of the hologram 132 inFIG. 5.

FIG. 6B is a cross-sectional view showing a configuration of thehologram 132 in FIG. 5.

FIG. 7A is a cross-sectional view showing a stepped shape in one period(p2) of a grating formed in the hologram 132.

FIG. 7B shows a phase modulation amount with respect to a blue lightbeam 61 (wavelength λ1) corresponding to FIG. 7A.

FIG. 7C shows a phase modulation amount with respect to a red light beam62 (wavelength λ2) corresponding to FIG. 7A.

FIG. 8A is a plan view showing a specific example of the hologram 13shown in FIG. 1 in Embodiment 3 of the present invention.

FIG. 8B is a cross-sectional view showing a specific example of thehologram 13 shown in FIG. 1 in Embodiment 3 according to the presentinvention.

FIG. 9A is a cross-sectional view showing a stepped shape in one period(p3) of a grating formed in an outer peripheral potion 133C of thehologram 133.

FIG. 9B shows a phase modulation amount with respect to the blue lightbeam 61 (wavelength λ1) corresponding to FIG. 9A.

FIG. 9C shows a phase modulation amount with respect to the red lightbeam 62 (wavelength λ2) corresponding to FIG. 9A.

FIG. 10 is a cross-sectional view showing a specific example of acomplex objective lens composed of the hologram 13 and the objectivelens 14 shown in FIG. 1 in Embodiment 4 of the present invention.

FIG. 11A is a plan view showing a configuration of a hologram 134 inFIG. 10.

FIG. 11B is a cross-sectional view showing a configuration of thehologram 134 in FIG. 10.

FIG. 12A is a cross-sectional view showing a sawtooth shape in oneperiod (p4) of a grating formed in the hologram 134.

FIG. 12B shows a phase modulation amount with respect to the blue lightbeam 61 (wavelength λ1) corresponding to FIG. 12A.

FIG. 12C shows a phase modulation amount with respect to the red lightbeam 62 (wavelength λ2) corresponding to FIG. 12A.

FIG. 13A is a cross-sectional view showing a sawtooth shape in oneperiod (p4) of a grating formed in an inner circumferential portion 134Cof the hologram 134 according to Embodiment 4 of the present invention.

FIG. 13B shows a phase modulation amount with respect to the blue lightbeam 61 (wavelength λ1) corresponding to FIG. 13A.

FIG. 13C shows a phase modulation amount with respect to the red lightbeam 62 (wavelength λ2) corresponding to FIG. 13A.

FIG. 14 is a cross-sectional view showing a specific example of acomplex objective lens composed of the hologram 13 and the objectivelens 14 shown in FIG. 1 in Embodiment 5 of the present invention.

FIG. 15A is a plan view showing a configuration of the hologram 135 inFIG. 14.

FIG. 15B is a cross-sectional view showing a configuration of thehologram 135 in FIG. 14.

FIG. 16A is a cross-sectional view showing a physical sawtooth shape inone period (p7) of a grating formed in an outer peripheral portion 135Bof the hologram 135.

FIG. 16B shows a phase modulation amount with respect to the blue lightbeam 61 (wavelength λ1) corresponding to FIG. 16A.

FIG. 16C shows a phase modulation amount with respect to the red lightbeam 62 (wavelength λ2) corresponding to FIG. 16A.

FIG. 17 is a graph showing a relationship between a depth “h4” of asawtooth grating formed in the inner circumferential portion 136C of thehologram 136 and a diffraction efficiency in Embodiment 7 of the presentinvention.

FIG. 18 is a cross-sectional view showing a specific example of acomplex objective lens in Embodiment 8 of the present invention.

FIG. 19 is a view showing a schematic configuration of an opticalinformation apparatus according to Embodiment 9 of the presentinvention.

FIG. 20 is a schematic view showing one exemplary configuration of acomputer according to Embodiment 10 of the present invention.

FIG. 21 is a schematic view showing one exemplary configuration of anoptical disk player according to Embodiment 11 of the present invention.

FIG. 22 is a schematic view showing one exemplary configuration of a carnavigation system according to Embodiment 12 of the present invention.

FIG. 23 is a schematic view showing one exemplary configuration of anoptical disk recorder according to Embodiment 13 of the presentinvention.

FIG. 24 is a schematic view showing one exemplary configuration of anoptical disk server according to Embodiment 14 of the present invention.

FIG. 25A is a cross-sectional view showing a schematic configuration ofan optical head for condensing 0th-order diffracted light 42 onto anoptical disk 10 with a base thickness of 0.6 mm in a first conventionalexample.

FIG. 25B is a cross-sectional view showing a schematic configuration ofan optical head for condensing +1st-order diffracted light 43 onto anoptical disk 11 with a base thickness of 1.2 mm in the firstconventional example.

FIG. 26 is a cross-sectional view showing a schematic configuration ofan optical head as a second conventional example.

FIG. 27A is a plan view showing a configuration of a wavelengthselection phase plate 205 in FIG. 26.

FIG. 27B is a cross-sectional view showing a configuration of thewavelength selection phase plate 205 in FIG. 26.

FIG. 28 is a cross-sectional view showing a schematic configuration ofan optical head as a sixth conventional example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described by way ofillustrative embodiments with reference to the drawings.

Embodiment 1

FIG. 1 is a cross-sectional view showing one exemplary configuration ofan optical head according to Embodiment 1 of the present invention. InFIG. 1, reference numeral 1 denotes a blue laser light source as a firstlaser light source for emitting a first light beam having a wavelengthλ1 (390 nm to 415 nm: in general, 405 nm is used often, so that awavelength of 390 nm to 415 nm will be collectively referred to as“about 405 nm”); 20 denotes a red laser light source as a second laserlight source for emitting a second light beam having a wavelength λ2(630 nm to 680 nm: in general, 660 nm is used often, so that awavelength of 630 nm to 680 nm will be collectively referred to as“about 660 nm”); 8 denotes a collimator lens; 12 denotes a rising mirrorthat bends an optical axis; 13 denotes a hologram (diffraction typeoptical element); and 14 denotes an objective lens as a refraction typelens. Herein, the hologram 13 and the objective lens 14 constitute acomplex objective lens in the present embodiment.

Reference numeral 9 denotes a BD (first optical disk) that is a thirdgeneration optical disk, which has a base thickness t1 of about 0.1 mm(a base thickness of 0.06 mm to 0.11 mm will be collectively referred toas “about 0.1 mm”) or less and with respect to which information isrecorded/reproduced with a first light beam having a wavelength λ1. 10denotes a second generation optical disk (second optical disk) such as aDVD, which has a base thickness t2 of about 0.6 mm (a base thickness of0.54 mm to 0.65 mm will be collectively referred to as “about 0.6 mm”)and with respect to which information is recorded/reproduced with asecond light beam having a wavelength λ2. Regarding the first opticaldisk 9 and the second optical disk 10, only a base from a light incidentsurface to a recording surface is shown. However, actually, in order toenhance mechanical strength and set the outer size to be 1.2 mm that isthe same size as that of a CD, a protective plate is attached to thefirst optical disk 9 and the second optical disk 10. A protective memberwith a thickness of 0.6 mm is attached to the second optical disk 10. Aprotective member with a thickness of 1.1 mm is attached to the firstoptical disk 9. In the figures referred to through the respectiveembodiments, the protective member is omitted for simplifyingillustration.

The blue laser light source 1 and the red laser light source 20 arepreferably semiconductor laser light sources. Because of this, anoptical head and an optical information apparatus using the same can beminiaturized and reduced in weight and power consumption.

When information is recorded/reproduced with respect to the firstoptical disk 9 with the highest recording density, a blue light beam 61with a wavelength λ1 emitted from the blue laser light source 1 isreflected by a beam splitter 4 and circularly polarized by a ¼wavelength plate 5. The ¼ wavelength plate 5 is designed so as tofunction as a ¼ wavelength plate with respect to both the blue lightbeam 61 with a wavelength λ1 and the red light beam 62 with a wavelengthλ2. The blue light beam 61 passing through the ¼ wavelength plate 5 issubstantially collimated by the collimator lens 8, has its optical axisbent by the rising mirror 12, and is condensed onto an informationrecording surface 91 (see FIG. 2) through the base with a thickness ofabout 0.1 mm of the first optical disk 9 by the hologram 13 and theobjective lens 14.

The blue light beam 61 reflected from the information recording surface91 follows the previous optical path in a reverse direction (returnpath), is linearly polarized in a direction orthogonal to the initialdirection by the ¼ wavelength plate 5, passes through the beam splitter4 substantially totally, is totally reflected by a beam splitter 16, isdiffracted by a detection hologram 31, has its focal length increased bya detection lens 32, and is incident upon a photodetector 33. Byoperating an output signal from the photodetector 33, a servo signal andan information signal used for focal point control and tracking controlare obtained.

As described above, the beam splitter 4 is a polarized light separationfilm that totally reflects linearly polarized light in one direction andtotally transmits linearly polarized light in a direction orthogonalthereto with respect to the blue light beam with a wavelength λ1.Furthermore, the beam splitter 4 totally transmits the red light beam 62emitted from the red laser light source 20 with respect to the red lightbeam with a wavelength λ2, as described later. Thus, the beam splitter 4is an optical path branching element having wavelength selectivitytogether with polarization characteristics.

Next, when information is recorded/reproduced with respect to the secondoptical disk 10, the red light beam 62 with a wavelength λ2, which issubstantially linearly polarized light emitted from the red laser lightsource 20, passes through the beam splitters 16 and 4, substantiallycollimated by the collimator lens 8, has its optical axis bent by therising mirror 12, and is condensed onto an information recording surface101 (see FIG. 2) through a base with a thickness of about 0.6 mm of thesecond optical disk 10 by the hologram 13 and the objective lens 14.

The red light beam reflected from the information recording surface 101follows the previous optical path in a reverse direction (return path),totally passes through the beam splitter 4, is totally reflected by thebeam splitter 16, is diffracted by the detection hologram 31, has itsfocal length increased by the detection lens 32, and is incident uponthe photodetector 33. By operating an output signal from thephotodetector 33, a servo signal and an information signal used forfocal point control and tracking control are obtained.

As described above, in order to obtain a servo signal for the firstoptical disk 9 and the second optical disk 10 from the commonphotodetector 33, the blue laser light source 1 and the red laser lightsource 20 are placed so that their lighting points have an image-formingrelationship with a common position on the objective lens 14 side.Because of this, the number of photodetectors and the number of wirescan be reduced.

The beam splitter 16 is a polarized light separation film that totallytransmits linearly polarized light in one direction and totally reflectslinear polarized light in a direction orthogonal thereto with respect tothe red light beam 62 with a wavelength λ2. The beam splitter 16 totallytransmits the blue light beam 61 with a wavelength λ1. Thus, the beamsplitter 16 also is an optical path branching element having wavelengthselectivity together with polarization characteristics, in the same wayas in the beam splitter 4.

FIG. 2 is a cross-sectional view showing a specific example of a complexobjective lens composed of the hologram 13 and the objective lens 14 inFIG. 1. In FIG. 2, reference numeral 131 denotes a hologram as adiffraction type optical element. The hologram 131 transmits a largelight amount of the blue light beam 61 with a wavelength λ1 withoutdiffracting it, and diffracts the red light beam 62 with a wavelengthλ2, as described later. Light that has not been diffracted when passingthrough a diffraction element is called 0th-order diffracted light, sothat such light will be represented as 0th-order diffracted light.

The hologram 131 transmits the blue light beam 61 with a wavelength λ1as 0th-order diffracted light, so that the hologram 131 does not converta wavefront with respect to the blue light beam 61. Thus, the objectivelens 141 is designed so that the substantially parallel blue light beam61 with a wavelength λ1 is condensed onto the information recordingsurface 91 through a base with a thickness t1 of the first optical disk9. Since the hologram 131 does not convert a wavefront with respect tothe blue light beam 61, it is not required to set the relative positionof the hologram 131 and the objective lens 141 with high precision interms of recording/reproducing of information with respect to the firstoptical disk 9. In the case where the permissible position error of theobjective lens 141 and the hologram 131 can be increased with respect tothe blue light beam, 61 with a wavelength λ1 for recording/reproducinginformation with respect to the first optical disk 9 with the shortestwavelength and the highest recording density, and as described later,information is recorded/reproduced with respect to an optical diskhaving a lower recording density with a light beam having a longerwavelength, the relative position of the hologram 131 and the objectivelens 141 may be considered. Thus, an optical head excellent inproductivity can be configured, in which the permissible margin of errorof the relative position can be increased more.

Next, the function of the hologram 131 when information isrecorded/reproduced with respect to the optical disk 10 using the redlight beam 62 will be described in detail. The hologram 131 transmitsthe blue light beam 61 with a wavelength λ1 as 0th-order light, anddiffracts the red light beam 62 with a wavelength λ2. The objective lens141 condenses the red light beam 62 onto the information recordingsurface 101 through a base with a thickness of about 0.6 mm of thesecond optical disk 10. Herein, in the second optical disk 10, the basethickness is large (i.e., 0.6 mm) from the light incident surface to theinformation recording surface 101. Therefore, it is required to set afocal point position farther away from the objective lens 141, comparedwith the focal point position in the case where information isrecorded/reproduced with respect to the first optical disk 9 with a basethickness of 0.1 mm. As shown in FIG. 2, the red light beam 62 isconverted to dispersed light by wavefront conversion, whereby the focalpoint position is corrected and spherical aberration caused by thedifference in base thickness is corrected.

The red light beam 62 with a wavelength λ2 is subjected to wavefrontconversion by the hologram 131. Thus, when there is an error in therelative position of the hologram 131 and the objective lens 141, thewavefront as designed is not incident upon the objective lens 141, andaberration occurs on the wavefront incident upon the second optical disk10, resulting in degraded condensing characteristics. Desirably, thehologram 131 and the objective lens 141 are integrally fixed by asupport 34, or the hologram 131 is formed directly on the surface of theobjective lens 141, whereby they are moved integrally by a commondriving unit 15 (FIG. 1) for focal point control and tracking control.

FIG. 3A is a plan view showing a configuration of the hologram 131, andFIG. 3B is a cross-sectional view similar to FIG. 2, showing aconfiguration of the hologram 131. The hologram 131 has differentconfigurations between an inner side (inner circumferential portion131C) and an outer side (outer peripheral portion 131B between aninner/outer peripheral boundary 131A and an effective range 131D) of theinner/outer peripheral boundary 131A. The inner circumferential portion131C is a region including a crossing point (i.e., the center) betweenthe hologram 131 and the optical axis. This region also is used forrecording/reproducing information with respect to the second opticaldisk 10 using the red light beam 62 and for recording/reproducinginformation with respect to the first optical disk 9 using the bluelight beam 61.

Thus, a concentric diffraction grating is formed in the innercircumferential portion 131C. Regarding the outer peripheral portion131B, it is required that a numerical aperture NAb when information isrecorded/reproduced with respect to the first optical disk 9 with theblue light beam 61 is larger than a numerical aperture NAr wheninformation is recorded/reproduced with respect to the second opticaldisk 10 with the red light beam 62 (NAb>NAr). Therefore, it is requiredto provide the outer peripheral portion 131B, which condenses only theblue light beam 61 onto the first optical disk 9 and allows the redlight beam 62 to have aberration with respect to the second optical disk10, around the inner circumferential portion 131C that condenses theblue light beam 61 and the red light beam 62 onto the respectivelycorresponding first optical disk 9 and second optical disk 10.

In the present embodiment, a hologram is not formed in the outerperipheral portion 131B. The objective lens 141 is designed so that theblue light beam 61 passing through the outer peripheral portion 131B iscondensed onto the first optical disk 9 after passing through a base ofabout 0.1 mm, whereby the red light beam 62 passing through the outerperipheral portion 131B is not condensed onto the second optical disk10, and the condition of NAb>NAr can be realized.

FIG. 4A is a cross-sectional view showing a physical level difference inone period (p1) of a grating formed in the inner circumferential portion131C of the hologram 131 shown in FIG. 3A, and FIG. 4B shows a phasemodulation amount with respect to the red light beam 62 (wavelength λ2)corresponding to FIG. 4A. Herein, the hologram 131 according to thepresent embodiment has a lens action, and a grating pitch is variedlocally. The grating pitch is illustrated merely using any points on thehologram 131 as representative. This also applies to the otherembodiments. In FIGS. 4A and 4B, a lower side represents a hologram baseside (side with a higher refractive index) and an upper side representsan air side (side with a lower refractive index). Hereinafter, the samedefinition will be used in similar figures.

In FIG. 4A, a vertical direction represents a level difference. Herein,a shape obtained by combining rectangular shapes will be referred to asa stepped shape. A “nb” represents a refractive index of a hologrammaterial with respect to the blue light beam 61 (wavelength λ1).Assuming that the hologram material is, for example, BK7, “nb” is1.5302. Herein, BK7 will be illustrated as an example. Other glassmaterials, polycarbonate, and polycycloolefin type resin materials alsocan be used. This also applies to the other embodiments.

It is assumed that one unit of the level difference corresponds to anamount at which a difference in optical path length is about onewavelength (i.e., phase difference is about 2π) with respect to the bluelight beam 61. Then, the unit level difference d1 is λ1/(nb−1)=0.764 μm.

When the level difference of a grating is set to be an integral multipleof the unit level difference d1 to obtain a stepped cross-sectionalshape, the phase modulation amount with respect to the blue light beam61 in this shape becomes an integral multiple of 2π, which substantiallycorresponds to the absence of phase modulation.

On the other hand, assuming that the refractive index of a hologrammaterial with respect to the red light beam 62 is “nr”, in the casewhere the hologram material is BK7, “nr” is 1.5142. Therefore, thedifference in optical path length occurring in the red light beam 62 dueto the unit level difference d1 is d1×(nr−1)/λ2=0.595, i.e., about 0.6times the wavelength λ2.

Assuming a stepped shape in which the level difference is set to be 0time, twice, once, and three times d1 from the right as shown in FIG.4A, as described above, phase modulation does not occur principally anddiffraction does not occur in the blue light beam 61, i.e., 0th-orderdiffracted light becomes strongest. Then, the difference in optical pathlength is changed in a stepped shape in the order of: 0 time, 1.2 times,0.6 times, and 1.8 times the wavelength λ2 with respect to the red lightbeam 62. Among them, the integral multiple corresponds to the absence ofphase modulation. Therefore, substantially, the difference in opticalpath length is changed in a stepped shape in the order of: 0 time, 0.2times, 0.6 times, and 0.8 times the wavelength λ2, as shown in FIG. 4B.The width of each step in one period is changed with respect to such astepped shape change, and a diffraction efficiency is calculated. As aresult, as shown in FIG. 4A, assuming that the ratio of step widths isabout 2:3:3:2, the diffraction efficiency of +1st-order diffracted lightof the red light beam 62 becomes highest, and about 75% diffractionefficiency is obtained according to the Scalar calculation.

The ratio of step widths herein is the ratio of physical lengths when aperipheral grating pitch is constant. However, when a peripheral gratingpitch is changed rapidly, it is desirable to change the ratio of stepwidths in accordance with the rapid change. This point also applies tothe embodiments described later. The stepped configuration that does notallow phase modulation to occur with respect to the blue light beam 61and diffracts the red light beam 62 also is disclosed in JP10(1998)-334504 A and Session We-C-05 of ISOM2001 (page 30 of thepreprints) listed as the second conventional example. However, thesedocuments do not show that the level difference is formed in a steppedshape in the order of 0 time, twice, once, and three times the unitlevel difference d1, and the ratio of step widths is set to be about2:3:3:2, as in the present embodiment.

In the configuration of the present example, the step difference is setto be at most three times the unit step difference d1, i.e. the stepdifference is minimized as a 4-stepped shape. Thus, the productionerrors and the loss of a light amount due to wall surfaces of the steps(rising surfaces in the vertical direction in the figure) are minimized.In addition, an optimum ratio of step widths is found. Thus, the lightamount of +1st-diffracted light of the red light beam 62 can beincreased, which is particularly advantageous for ensuring a recordinglight amount. However, such an effect cannot be obtained in the priorart.

Furthermore, with respect to the overall configuration of an opticalhead, an example of an additionally effective configuration will bedescribed below. The following configuration is effective for all theembodiments. The important point of the present embodiment lies in thehologram 13 (131 in the present embodiment) for realizing compatiblereproducing/recording of information with respect to the first andsecond optical disks 9 and 10, and the objective lens 14 (141 in thepresent embodiment) used in combination with the hologram 13. In theother configurations described above and below, the beam splitter 16,the detection lens 32, and the detection hologram 31 are not requiredelements. These elements exhibit their effects as preferableconfigurations; however, the other configurations can be usedappropriately.

In FIG. 1, by placing a 3-beam grating (diffraction element) 3 betweenthe blue laser light source 1 and the beam splitter 4, a tracking errorsignal of the first optical disk 9 can be detected by a well-knowndifferential push-pull (DPP) method.

Furthermore, in the case where two directions vertical to an opticalaxis are defined as an x-direction and a y-direction, by placing a beamshaping element 2 that enlarges, for example, only an x-directionbetween the blue laser light source 1 and the beam splitter 4, afar-field pattern of the blue light beam 61 can be approximated to anintensity distribution close to a point-symmetrical system with respectto an optical axis, and a light use efficiency can be enhanced The beamshaping element 2 can be configured by using a double-sided cylindricallens.

By further placing the 3-beam grating (diffraction element) 22 betweenthe red laser light source 20 and the beam splitter 16, a tracking errorsignal of the second optical disk 10 can be detected by a well-knowndifferential push-pull (DPP) method.

Furthermore, it also is effective to change the parallel degree of alight beam by moving the collimator lens 8 in an optical axis direction(right-left direction in FIG. 1). If there is a thickness error of abase, and a base thickness ascribed to an interlayer thickness in thecase where the first optical disk 9 is a two-layer disk, sphericalaberration occurs. However, by moving the collimator lens 8 in anoptical axis direction, spherical aberration can be corrected. Thus, thespherical aberration can be corrected by about several 100 mλ when anumerical aperture NA of light condensed onto the first optical disk 9is 0.85, by moving the collimator lens 8, whereby a base thickness of±30 μm can be corrected.

However, when information is recorded/reproduced with respect to a DVDusing the objective lens 14 corresponding to a base thickness of 0.1 mm,it is required to compensate for a difference in base thickness of 0.5mm or more. In this case, the correction of spherical aberration onlywith the movement of the collimator lens 8 is insufficient, so that awavefront needs to be converted by the hologram (131 for example). Thefollowing also is possible: in the case where information isrecorded/reproduced with respect to the second optical disk 10 using thered light beam 62, the collimator lens 8 is moved to the left side ofFIG. 1, i.e., the side close to the red laser light source 20. Becauseof this, the red light beam 62 directed to the objective lens 14 isconverted to dispersed light, and a spot condensed onto the secondoptical disk 10 is placed away from the objective lens 14. In addition,a part of aberration caused by a base thickness is corrected to reducean aberration correction amount required for the hologram 13.Accordingly, a hologram pitch is enlarged to facilitate the productionof the hologram 13.

Furthermore, the beam splitter 4 is configured so as to transmit a part(e.g., about 10%) of linearly polarized light emitted from the bluelaser light source 1, and the transmitted light beam is guided to aphotodetector 7 by a condensing lens 6. Accordingly, a change in theamount of light emitted from the blue laser light source 1 is monitoredby using a signal obtained from the photodetector 7, and the change inlight amount is fed back, whereby the amount of light emitted from theblue laser light source 1 can be controlled to be constant.

Furthermore, the beam splitter 4 is configured so as to reflect a part(e.g., about 10%) of linearly polarized light emitted from the red laserlight source 20, and the reflected light beam is guided to thephotodetector 7 by the condensing lens 6. Accordingly, a change in theamount of light emitted from the red laser light source 20 is monitoredby using a signal obtained from the photodetector 7, and the change inlight amount is fed back, whereby the amount of light emitted from thered laser light source 20 can be controlled to be constant.

Furthermore, as shown in FIG. 2, in order to set a numerical aperture(NA), when the blue light beam 61 is condensed onto the first opticaldisk 9, to be a desired value (about 0.85), it is effective to providean opening limitation member 341. Particularly, in the case where theobjective lens 141 and the hologram 131 are fixed integrally using thesupport 34, and they are moved by the driving unit 15 (FIG. 1), if theshape of the support 34 is formed, for example, as shown in FIG. 2, sothat the opening limitation member 341 is formed integrally, the numberof parts can be reduced.

Furthermore, in FIG. 2, by cutting a portion of the objective lens 14(141 for example), which is on a side dose to the second optical disk 10and through which the blue light beam 61 does not pass due to thedistance from an optical axis (forming a cut-away portion 1411), or byforming the objective lens 14 so as not to include a member in part,when information is recorded/reproduced with respect to an optical diskin a cartridge, the objective lens 14 can be prevented from coming intocontact with the cartridge.

Embodiment 2

Next, Embodiment 2 of the present invention will be described. Theoverall configuration of an optical head according to the presentembodiment is the same as that shown in FIG. 1 referred to in thedescription of Embodiment 1. In the present embodiment, theconfiguration of the hologram 13 and the objective lens 14 shown in FIG.1 is different from that of Embodiment 1.

FIG. 5 is a cross-sectional view showing a specific example of a complexobjective lens composed of the hologram 13 and the objective lens 14shown in FIG. 1. In FIG. 5, reference numeral 132 denotes a hologram.The hologram 132 diffracts the blue light beam 61 with a wavelength λ1to effect a convex lens action, and diffracts the red light beam 62 witha wavelength λ2 to effect a concave lens action, as described later.Herein, the diffraction of the lowest order that effects a convex lensaction is defined as +1st-order diffraction. Then, the red light beam 62is subjected to a concave lens action by −1st-order diffraction that isconjugate to +1st-order diffracted light, i.e., that has a diffractiondirection at each point on the hologram 132 opposite to that of+1st-order diffracted light.

An objective lens 142 is designed in such a manner that after the bluelight beam 61 with a wavelength λ1 is diffracted by the hologram 132 andsubjected to a convex lens action, the objective lens 142 converges theblue light beam 61 so as to condense it onto an information recordingsurface 91 through a base of the first optical disk 9 with a thicknessof about 0.1 mm.

Next, the function of the hologram 132 when information isrecorded/reproduced with respect to the second optical disk 10 using thered light beam 62 will be described. The hologram 132 diffracts the redlight beam 62 with a wavelength λ2 as −1st-order diffracted light toeffect a concave lens action. The objective lens 142 condenses the redlight beam 62 onto the information recording surface 101 through a baseof the second optical disk 10 with a thickness of about 0.6 mm. Herein,in the second optical disk 10, the base thickness is large (i.e., 0.6mm) from the light incident surface to the information recording surface101. Therefore, it is required to set a focal point position fartheraway from the objective lens 142, compared with the focal point positionin the case where information is recorded/reproduced with respect to thefirst optical disk 9 with a base thickness of 0.1 mm. As shown in FIG.5, the blue light beam 61 is converted to converged light and the redlight beam 62 is converted to dispersed light by wavefront conversion,whereby the focal point position is corrected and spherical aberrationcaused by the difference in base thickness is corrected.

The blue light beam 61 with a wavelength λ1 and the red light beam 62with a wavelength λ2 are subjected to wavefront conversion by thehologram 132. Thus, when there is an error in the relative position ofthe hologram 132 and the objective lens 142, the wavefront as designedis not incident upon the objective lens 142, and aberration occurs onthe wavefront incident upon the first optical disk 9 and the secondoptical disk 10, resulting in degraded condensing characteristics.Desirably, the hologram 132 and the objective lens 142 are integrallyfixed, whereby they are moved integrally by the common driving unit 15(FIG. 1) for focal point control and tracking control.

FIG. 6A is a plan view showing a configuration of the hologram 132, andFIG. 6B is a cross-sectional view similar to FIG. 5, showing aconfiguration of the hologram 132. The hologram 132 has differentconfigurations between an inner side (inner circumferential portion132C) and an outer side (outer peripheral portion 132B between aninner/outer peripheral boundary 132A and an effective range 132D) of theinner/outer peripheral boundary 132A. The inner circumferential portion132C is a region including a crossing point (i.e., the center) betweenthe hologram 132 and the optical axis. This region also is used forrecording/reproducing information with respect to the second opticaldisk 10 using the red light beam 62 and for recording/reproducinginformation with respect to the first optical disk 9 using the bluelight beam 61. Thus, a diffraction grating in the inner circumferentialportion 132C and a portion of the objective lens 142 through which thered light beam 62 diffracted from the diffraction grating passes aredesigned so that +1st-order diffracted light of the blue light beam 61is condensed onto the first optical disk 9, and −1st-order diffractedlight of the red light beam 62 is condensed onto the second optical disk10.

Regarding the outer peripheral portion 132B, it is required that anumerical aperture NAb when information is recorded/reproduced withrespect to the first optical disk 9 with the blue light beam 61 islarger than a numerical aperture NAr when information isrecorded/reproduced with respect to the second optical disk 10 with thered light beam 62 (NAb>NAr). Therefore, it is required to provide theouter peripheral portion 132B and an outer peripheral portion of theobjective lens 142 corresponding thereto, so as to condense only+1st-order diffracted light of the blue light beam 61 onto the firstoptical disk 9 and allow −1st-order diffracted light of the red lightbeam 62 to have aberration with respect to the second optical disk 10,around the inner circumferential portion that condenses the blue lightbeam 61 and the red light beam 62 onto the respectively correspondingfirst optical disk 9 and second optical disk 10. More specifically,although not shown, it is desirable to design the objective lens 142differently between the inner and outer peripheries, in the same way asin the hologram 132. Because of this, an optimum NA, i.e., the conditionof NAb>NAr can be realized.

FIG. 7A is a cross-sectional view showing a physical level difference inone period (p2) of a grating formed in the hologram 132. FIG. 7B shows aphase modulation amount with respect to the blue light beam 61(wavelength λ1) corresponding to FIG. 7A. FIG. 7C shows a phasemodulation amount with respect to the red light beam 62 (wavelength λ2)corresponding to FIG. 7A.

In FIG. 7A, a vertical direction represents a level difference. A “nb”represents a refractive index of a hologram material with respect to theblue light beam 61. Assuming that the hologram material is, for example,BK7, “nb” is 1.5302. It is assumed that one unit of the level differencecorresponds to an amount at which a difference in optical path length isabout 1.25 wavelengths (i.e., phase difference is about 2π+π/2) withrespect to the blue light beam. Then, the unit level difference d2 is1.25×λ1/(nb−1)=0.955 μm.

When the level difference of a grating is set to be an integral multipleof the unit level difference d2 to obtain a stepped cross-sectionalshape in which the ratio of four step widths is 1:1:1:1, the phasemodulation amount with respect to the blue light beam 61 in this shapebecomes an integral multiple of 2π+π/2, which substantially correspondsto the phase modulation amount of π/2 per step.

On the other hand, assuming that the refractive index of a hologrammaterial with respect to the red light beam 62 is “nr”, in the casewhere the hologram material is BK7, “nr” is 1.5142. Therefore, thedifference in optical path length occurring in the red light beam 62 dueto the unit level difference d2 is d2×(nr−1)/λ2=0.744, i.e., about ¾times the wavelength λ2, and the phase modulation amount becomes about−π/2 per step.

A stepped cross-sectional shape of 4 steps is assumed in which the leveldifference of a grating is an integral multiple of the unit leveldifference d2, as shown in FIG. 7A. When a level difference is continuedto be overlaid, as shown in FIG. 7B, the phase modulation amount ischanged by π/2 per step, i.e., the difference in optical path length ischanged by +0.25 times λ1 with respect to the blue light beam 61. Whenthe physical shape of the level difference is formed as shown in FIG.7A, in the blue light beam 61, the diffraction efficiency of +1st-orderdiffracted light being subjected to a convex lens action is calculatedto be about 80% (Scalar calculation), which is strongest among thediffraction orders.

When a level difference is continued to be overlaid, as shown in FIG.7C, the phase modulation amount is changed by −π/2 per step, i.e., thedifference in optical path length is changed by −0.25 times λ2 withrespect to the red light beam 62. When the physical shape of the leveldifference is formed as shown in FIG. 7A, in the red light beam 62, thediffraction efficiency of −1st-order diffracted light being subjected toa concave lens action is calculated to be about 80% (Scalarcalculation), which is strongest among the diffraction orders.

As described in the present embodiment, because of the hologramconfiguration having a stepped cross-sectional shape that causes thedifference in optical path length that is 1.25 times the wavelength perstep, the compatible recording and reproducing of information withrespect to different kinds of disks can be realized, which use+1st-order diffracted light and −1st-order diffracted light respectivelyhaving a diffraction efficiency of 50% or more. None of theabove-mentioned conventional examples disclose such compatible recordingand reproducing.

In the present embodiment, because of the above-mentioned novelconfiguration, the diffraction order of the blue light beam 61 and thered light beam 62 become +1st-order diffracted light and −1st-orderdiffracted light, respectively, and the difference in order becomes 2.Therefore, a minimum pitch of a hologram required for exhibiting thesame aberration correction effect and the movement effect of a focalpoint position can be made larger than that of Embodiment 1, and ahologram can be produced easily. Furthermore, the amount of diffractedlight as calculated can be obtained easily.

Furthermore, the hologram 132 has a convex lens action with respect tothe blue light beam 61. The chromatic dispersion by the diffractionaction is in an opposite direction to that of the refraction action.Therefore, when the hologram 132 is combined with the objective lens 142that is a refraction type convex lens, chromatic aberration with respectto a wavelength change within several nm, in particular, wavelengthdependency of a focal length can be cancelled.

Furthermore, the overall configuration of the optical head can becombined with the configuration additionally described in Embodiment 1.

Embodiment 3

Next, Embodiment 3 of the present invention will be described. Theoverall configuration of an optical head according to the presentembodiment is the same as that shown in FIG. 1 referred to in thedescription of Embodiment 1. In the present embodiment, theconfiguration of the hologram 13 shown in FIG. 1 is different from thoseof Embodiments 1 and 2.

FIGS. 8A and 8B are a plan view and a cross-sectional view showing aspecific example of the hologram 13 shown in FIG. 1, respectively. InFIGS. 8A and 8B, reference numeral 133 denotes a hologram. An innercircumferential portion 133C of the hologram 133 is the same as theinner circumferential portion 132C of the hologram 132 illustrated anddescribed in Embodiment 2. Furthermore, a grating pitch of an outerperipheral portion 133B also is the same as that of the outer peripheralportion 132B of the hologram 132 illustrated and described in Embodiment2. However, as shown in FIG. 8B, the cross-sectional shape of a gratingformed in the outer peripheral portion 133B is different from that inthe outer peripheral portion 132B.

FIG. 9A is a cross-sectional view showing a physical level difference inone period (p3) of a grating formed in the outer peripheral portion 133Bof the hologram 133. FIG. 9B shows a phase modulation amount withrespect to the blue light beam 61 (wavelength λ1) corresponding to FIG.9A. FIG. 9C shows a phase modulation amount with respect to the redlight beam 62 (wavelength λ2) corresponding to FIG. 9A.

In FIG. 9A, a vertical direction represents a level difference. A “nb”represents a refractive index of a hologram material with respect to theblue light beam 61. Assuming that the hologram material is, for example,BK7, “nb” is 1.5302.

It is assumed that one unit of the level difference corresponds to anamount at which a difference in optical path length is about 0.25wavelengths (i.e., phase difference is about π/2) with respect to theblue light beam. Then, the unit level difference d3 is0.25×λ1/(nb−1)=0.191 μm.

On the other hand, assuming that the refractive index of a hologrammaterial with respect to the red light beam 62 is “nr”, in the casewhere the hologram material is BK7, “nr” is 1.5142. Therefore, thedifference in optical path length occurring in the red light beam 62 dueto the unit level difference d3 is d3×(nr−1)/λ2=0.149, i.e., about 0.15times the wavelength λ2, and the phase modulation amount becomes about0.3π per step.

The level difference of a grating is set to be an integral multiple ofthe unit level difference d3 to obtain a stepped cross-sectional shapein which the ratio of four step widths is about 1:1:1:1, as shown inFIG. 9A. When a level difference is continued to be overlaid, as shownin FIG. 9B, the phase modulation amount is changed by π/2 per step,i.e., the difference in optical path length is changed by +0.25 times λ1with respect to the blue light beam 61. When the physical shape of thelevel difference is formed as shown in FIG. 9A, in the blue light beam61, the diffraction efficiency of +1st-order diffracted light beingsubjected to a convex lens action is calculated to be about 80% (Scalarcalculation), which is strongest among the diffraction orders.

When a level difference is continued to be overlaid, as shown in FIG.9C, the phase modulation amount is changed by −0.3π per step, i.e., thedifference in optical path length is changed by 0.15 times λ2 withrespect to the red light beam 62. When the physical shape of the leveldifference is formed as shown in FIG. 9A, in the red light beam 62, thediffraction efficiency of +1st-order diffracted light being subjected toa convex lens action is calculated to be about 50% (Scalar calculation),which is strongest among the diffraction orders. This is the same orderas that of the blue light beam 61. Therefore, the red light beam 62undergoes large aberration with respect to the second optical disk 10,and is not condensed thereon. Furthermore, in the red light beam 62, thediffraction efficiency of −1st-order diffracted light being subjected toa concave lens action is sufficiently weak (i.e., 10% or less). Thus, bydecreasing the numerical aperture of the red light beam 62 with respectto the second optical disk 10, the condition (NAb>NAr) can be realizedeasily, under which the numerical aperture NAb when information isrecorded/reproduced with respect to the first optical disk 9 with theblue light beam 61 is larger than the numerical aperture NAr wheninformation is recorded/reproduced with respect to the second opticaldisk 10 with the red light beam 62.

As described above, in the present embodiment, a hologram has aconfiguration in which only the outer peripheral portion 133B has astepped cross-sectional shape causing a difference in optical path thatis 0.25 times the wavelength per step. The inner circumferential portion133C uses conjugate light. None of the above-mentioned conventionalexamples disclose such compatible recording and reproducing ofinformation with respect to different kinds of disks.

In the present embodiment, because of the above-mentioned novelconfiguration, in addition to the advantage described in Embodiment 2,the height of a grating in the outer peripheral portion 133B, in which agrating pitch of the hologram 133 is relatively narrow, can be lowered.Therefore, the hologram 133 can be produced easily. Furthermore, bydecreasing the numerical aperture of the red light beam 62 with respectto the second optical disk 10, the condition (NAb>NAr) can be realizedeasily, under which the numerical aperture NAb when information isrecorded/reproduced with respect to the first optical disk 9 with theblue light beam 61 is larger than the numerical aperture NAr wheninformation is recorded/reproduced with respect to the second opticaldisk 10 with the red light beam 62.

Furthermore, the overall configuration of the optical head can becombined with the configuration additionally described in Embodiment 1.

Embodiment 4

Next, Embodiment 4 of the present invention will be described. Theoverall configuration of an optical head according to the presentembodiment is the same as that shown in FIG. 1 referred to in thedescription of Embodiment 1. In the present embodiment, theconfiguration of the hologram 13 and the objective lens 14 shown in FIG.1 is different from those of Embodiments 1 to 3.

FIG. 10 is a cross-sectional view showing a specific example of acomplex objective lens composed of the hologram 13 and the objectivelens 14 shown in FIG. 1. In FIG. 10, reference numeral 134 denotes ahologram. The hologram 134 diffracts the blue light beam 61 with awavelength λ1 to effect a convex lens action, and diffracts the redlight beam 62 with a wavelength λ2 to effect a convex lens action weakerthan that with respect to the blue light beam 61, as described later.Herein, the lowest order diffraction that effects a convex lens actionis defined as 1st-order diffraction. In the present embodiment, thehologram 134 is designed so that +2nd-order diffraction is effected moststrongly with respect to the blue light beam 61. Because of this,+1st-order diffraction is effected most strongly with respect to the redlight beam 62. Consequently, irrespective of whether the red light beam62 has a wavelength longer than that of the blue light beam 61, adiffraction angle at each point on the hologram 134 becomes small. Morespecifically, the convex lens action of the hologram 134 when thehologram 134 diffracts the blue light beam 61 with a wavelength λ1becomes stronger than that with respect to the red light beam 62 with awavelength λ2. In other words, although the red light beam 62 issubjected to a convex lens action by the hologram 134, the red lightbeam 62 is relatively subjected to a concave lens action by diffraction,with reference to the action with respect to the blue light beam 61.

The objective lens 144 is designed as follows: after the hologram 134subjects the blue light beam 61 with a wavelength λ1 to +2nd-orderdiffraction to effect a convex lens action, the objective lens 144further converges the blue light beam 61 to condense it onto theinformation recording surface 91 through a 0.1 mm thick base of thefirst optical disk 9.

Next, the function of the hologram 134 when information isrecorded/reproduced with respect to the second optical disk 10 with thered light beam 62 will be described in detail. The hologram 134 subjectsthe red light beam 62 with a wavelength λ2 to +1st-order diffraction toeffect a convex lens action. The objective lens 144 condenses the redlight beam 62 onto the information recording surface 101 through a 0.6mm thick base of the second optical disk 10. Herein, in the secondoptical disk 10, the base thickness is large (i.e., 0.6 mm) from thelight incident surface to the information recording surface 101.Therefore, it is required to set a focal point position farther awayfrom the objective lens 144, compared with the focal point position inthe case where information is recorded/reproduced with respect to thefirst optical disk 9 with a base thickness of 0.1 mm. As shown in FIG.10, the blue light beam 61 is converted to converged light by wavefrontconversion, and the conversion degree of the red light beam 62 is set tobe lower than that of the blue light beam 61, whereby the focal pointposition is corrected and spherical aberration caused by the differencein base thickness is corrected.

The blue light beam 61 with a wavelength λ1 and the red light beam 62with a wavelength λ2 are subjected to wavefront conversion by thehologram 134. Thus, when there is an error in the relative position ofthe hologram 134 and the objective lens 144, the wavefront as designedis not incident upon the objective lens 144, and aberration occurs onthe wavefront incident upon the first optical disk 9 and the secondoptical disk 10, resulting in degraded condensing characteristics.Desirably, the hologram 134 and the objective lens 144 are integrallyfixed, whereby they are moved integrally by the common driving unit 15(FIG. 1) for focal point control and tracking control.

FIG. 11A is a plan view showing a configuration of the hologram 134, andFIG. 11B is a cross-sectional view similar to FIG. 10, showing aconfiguration of the hologram 134. The hologram 134 has differentconfigurations between an inner side (inner circumferential portion134C) and an outer side (outer peripheral portion 134B between aninner/outer peripheral boundary 134A and an effective range 134D) of theinner/outer peripheral boundary 134A The inner circumferential portion134C is a region including a crossing point (i.e., the center) betweenthe hologram 134 and the optical axis. This region also is used forrecording/reproducing information with respect to the second opticaldisk 10 using the red light beam 62 and for recording/reproducinginformation with respect to the first optical disk 9 using the bluelight beam 61. Thus, a diffraction grating in the inner circumferentialportion 134C and a portion of the objective lens 144 through which thered light beam 62 diffracted from the diffraction grating passes aredesigned so that +2nd-order diffracted light of the blue light beam 61is condensed onto the first optical disk 9, and +1st-order diffractedlight of the red light beam 62 is condensed onto the second optical disk10.

Regarding the outer peripheral portion 134B, it is required that anumerical aperture NAb when information is recorded/reproduced withrespect to the first optical disk 9 with the blue light beam 61 islarger than a numerical aperture NAr when information isrecorded/reproduced with respect to the second optical disk 10 with thered light beam 62 (NAb>NAr). Therefore, it is required to provide theouter peripheral portion 132B and an outer peripheral portion of theobjective lens 144 corresponding thereto, so as to condense only+2nd-order diffracted light of the blue light beam 61 onto the firstoptical disk 9 and allow +1st-order diffracted light of the red lightbeam 62 to have aberration with respect to the second optical disk 10,around the inner circumferential portion that condenses the blue lightbeam 61 and the red light beam 62 onto the respectively correspondingfirst optical disk 9 and second optical disk 10. More specifically,although not shown, it is desirable to design the objective lens 144differently between the inner and outer peripheries, in the same way asin the hologram 134. Because of this, an optimum NA, i.e., the conditionof NAb>NAr can be realized.

FIG. 12A is a cross-sectional view showing a physical shape in oneperiod (p4) of a grating formed in the hologram 134. FIG. 12B shows aphase modulation amount with respect to the blue light beam 61(wavelength λ1) corresponding to FIG. 12A. FIG. 12C shows a phasemodulation amount with respect to the red light beam 62 (wavelength λ2)corresponding to FIG. 12A.

In FIG. 12A, the physical cross-sectional shape in one period (p4) ofthe grating has a sawtooth cross-sectional shape. Herein, in order torepresent the direction of a slope in the sawtooth cross-sectionalshape, the cross-sectional shape in FIG. 12A is expressed as across-sectional shape with a base having a slope on the left side. Inaccordance with this designation, the cross-sectional shape of thehologram 134 shown in FIG. 11B is expressed as a sawtoothcross-sectional shape (or simply a sawtooth shape) with a base having aslope on an outer peripheral side.

In FIG. 12A, the vertical direction represents the depth of a gratinghaving a sawtooth cross-sectional shape. A “nb” represents a refractiveindex of a hologram material with respect to the blue light beam 61.Assuming that the hologram material is, for example, BK7, “nb” is1.5302.

It is assumed that a depth “h1” of the sawtooth grating corresponds toan amount at which a difference in optical path length is about 2wavelengths (i.e., phase difference is about 4π) with respect to theblue light beam 61. Then, h1=2×λ1/(nb−1)=1.53 μm.

The phase modulation amount with respect to the blue light beam 61 inthe above-mentioned shape is changed by 4π (=2×2π) in one period of thegrating. Therefore, the intensity of +2nd-order diffracted light ismaximized with respect to the blue light beam 61, and the diffractionefficiency based on the Scalar calculation becomes 100%.

On the other hand, assuming that the refractive index of a hologrammaterial with respect to the red light beam 62 is “nr”, in the casewhere the hologram material is BK7, “nr” is 1.5142. Therefore, thedifference in optical path length occurring in the red light beam 62 dueto the depth “h1” is h1×(nr−1)/λ2=1.19, i.e., about 1.2 times thewavelength λ2, and the phase modulation amount becomes about 2.4π. Thus,the intensity of +1st-order diffracted light becomes highest withrespect to the red light beam 62, and the diffraction efficiency basedon the Scalar calculation becomes about 80%.

It is assumed that the shape of a grating in one period is a sawtoothcross-sectional shape with a depth “h1”, as shown in FIG. 12A. Since+2nd-order diffraction is strongest with respect to the blue light beam61, as described above, the grating period determining a diffractionangle is substantially p4/2, and a phase change becomes as shown in FIG.12B. With respect to the red light beam 62, +1st-order diffraction isstrongest, so that the grating period determining a diffraction angle issubstantially p4.

As described above, in the present embodiment, compatible recording andreproducing of information with respect to different kinds of disks isrealized with +1st-order diffracted light of the red light beam 62,using a hologram having a sawtooth cross-sectional shape with a depthcausing the difference in optical path length that is twice thewavelength λ1 and effects +2nd-order diffraction with respect to theblue light beam 61. None of the above-mentioned conventional examplesdisclose this concept.

In the present embodiment, because of the above-mentioned novelconfiguration, the hologram 134 has a convex lens action with respect toboth the blue light beam 61 and the red light beam 62. The chromaticdispersion by the diffraction action is in an opposite direction to thatof the refraction action. Therefore, when the hologram 134 is combinedwith the objective lens 144 that is a refraction type convex lens,chromatic aberration with respect to a wavelength change within severalnm, in particular, wavelength dependency of a focal length can becancelled.

Thus, according to the present embodiment, there is a remarkable effectthat three objects: compatibility among different kinds of disks,correction of chromatic aberration, and correction of a focal pointposition can be achieved with only the hologram 134.

Embodiment 5

Next, Embodiment 5 of the present invention will be described. In thepresent embodiment, only a cross-sectional shape of a grating formed inthe inner circumferential portion 134C of the hologram 134 of Embodiment4 is changed.

FIG. 13A is a cross-sectional view showing a sawtooth shape in oneperiod (p4) of a grating formed in the inner circumferential portion134C of the hologram 134 according to the present embodiment. FIG. 13Bshows a phase modulation amount with respect to the blue light beam 61(wavelength λ1) corresponding to FIG. 13A. FIG. 13C shows a phasemodulation amount with respect to the red light beam 62 (wavelength λ2)corresponding to FIG. 13A.

In FIG. 13A, a vertical direction represents the depth of the sawtoothgrating. In the present embodiment, unlike Embodiment 4, the depth isdetermined based on the red light beam 62. A “nr” represents arefractive index of a hologram material with respect to the red lightbeam 62. Assuming that the hologram material is, for example, BK7, “nr”is 1.5142.

It is assumed that a depth “h2” of the sawtooth grating corresponds toan amount at which a difference in optical path length is about onewavelength (i.e., phase difference is about 2π) with respect to the redlight beam 62. Then, h2=λ2/(nr−1)=1.28 μm.

On the other hand, assuming that the refractive index of a hologrammaterial with respect to the blue light beam 61 is “nb”, in the casewhere the hologram material is BK7, “nb” is 1.5302. Therefore, thedifference in optical path length occurring in the blue light beam 61due to the depth “h2” of the sawtooth grating is h2×(nb−1)/λ1=1.68,i.e., about 1.7 times the wavelength λ1, and the phase modulation amountbecomes about 3.35π. Thus, the intensity of +2nd-order diffracted lightbecomes highest with respect to the blue light beam 61, and thediffraction efficiency based on the Scalar calculation becomes about80%.

It is assumed that the shape of a grating in one period is a sawtoothcross-sectional shape with a depth “h2”, as shown in FIG. 13A. Since+2nd-order diffraction is strongest with respect to the blue light beam61, as described above, the grating period determining a diffractionangle is substantially p4/2, and a phase change becomes as shown in FIG.13B. The phase modulation amount per period p4 of the shape shown inFIG. 13A is about 3.35π. Therefore, as shown in FIG. 13B, consideringthe difference in optical path length for substantially one period p4/2,the phase modulation amount is 0.83 times the wavelength λ1 to be about1.7π. The intensity of +1st-order diffracted light is maximized withrespect to the red light beam 62, and the diffraction efficiency basedon the Scalar calculation is 100%. Thus, a light use efficiency can beenhanced.

Furthermore, the diffraction efficiency of +2nd-order diffracted lightof the blue light beam 61 is decreased to about 80%; however, when thelight amount at the central portion is decreased, the light amount inthe outer peripheral portion is increased relatively. The intensity of afar-field pattern of a semiconductor laser light source is decreasedtoward the outer peripheral portion, and only a part thereof can beused. When the light amount in the inner circumferential portion isdecreased, a larger range of the far-field pattern can be used; as aresult, a light use efficiency can be enhanced. This can be realized byshortening the focal length of the collimator lens 8. Because of this, adecrease in a light amount in the inner circumferential portion can becompensated.

Thus, according to the present embodiment, as described with referenceto FIG. 13A, by forming the inner circumferential portion of thehologram 134 as a sawtooth grating with a depth “h2”, the intensity ofdiffraction light of the red light beam 62 can be maximized. At thistime, a light use efficiency with respect to a condensed spot of theblue light beam 61 is not decreased.

In the present embodiment, the hologram 134 also has a convex lensaction with respect to both of the blue light beam 61 and the red lightbeam 62. The chromatic dispersion by the diffraction action is in anopposite direction to that of the refraction action. Therefore, when thehologram 134 is combined with the objective lens 144 that is arefraction type convex lens, chromatic aberration with respect to awavelength change within several nm, in particular wavelength dependencyof a focal length, can be cancelled.

Thus, according to the present embodiment, there is a remarkable effectthat three objects: compatibility among different kinds of disks,correction of chromatic aberration, and correction of a focal pointposition can be achieved only with the hologram 134.

Furthermore, a lens with a high NA is likely to be difficult to produce.However, by allowing the hologram 134 to have a convex lens action, thedifficulty in producing the refraction type objective lens 144 to becombined can be reduced.

Furthermore, the overall configuration of the optical head can becombined with the additionally described configuration in Embodiment 1.

Embodiment 6

Next, Embodiment 6 of the present invention will be described. Theoverall configuration of an optical head according to the presentembodiment is the same as that shown in FIG. 1 referred to in thedescription of Embodiment 1. In the present embodiment, theconfiguration of the hologram 13 shown in FIG. 1 is different from thoseof Embodiments 1 to 5.

FIG. 14 is a cross-sectional view showing a specific example of acomplex objective lens composed of the hologram 13 and the objectivelens 14 shown in FIG. 1. FIG. 15A is a plan view showing a configurationof the hologram 135, and FIG. 15B is a cross-sectional view similar toFIG. 14, showing a configuration of the hologram 135.

In FIGS. 14, 15A, and 15B, reference numeral 135 denotes a hologram. InFIG. 15A, an inner circumferential portion 135C of the hologram 135 has,for example, the same configuration as that of the inner circumferentialportion 134C of the hologram 134 according to Embodiment 4 or 5. Herein,the inner circumferential portion 135C of the hologram 135 may have anyconfiguration shown in Embodiments 1 to 5. However, when the innercircumferential portion 135C has the same configuration as that of theinner circumferential portion 134C of the hologram 134, there is anadvantage that the hologram 135 can be produced more easily because ofthe similarity of a sawtooth shape.

FIG. 16A is a cross-sectional view showing a physical sawtooth shape inone period (p7) of a grating formed in the outer peripheral portion 135Bof the hologram 135 according to the present embodiment. FIG. 16B showsa phase modulation amount with respect to the blue light beam 61(wavelength λ1) corresponding to FIG. 16A. FIG. 16C shows a phasemodulation amount with respect to the red light beam 62 (wavelength λ2)corresponding to FIG. 16A.

In FIG. 16A, a vertical direction represents the depth of the sawtoothshape. A “nb” represents a refractive index of a hologram material withrespect to the blue light beam 61. Assuming that the hologram materialis, for example, BK7, “nb” is 1.5302.

It is assumed that a depth “h3” of the sawtooth shape corresponds to anamount at which a difference in optical path length is about onewavelength (FIG. 16B), i.e., the phase difference is about 2π withrespect to the blue light beam 61. Then, h3=λ1/(nb−1)=0.764 μm.

On the other hand, assuming that the refractive index of a hologrammaterial with respect to the red light beam 62 is “nr”, in the casewhere the hologram material is BK7, “nr” is 1.5142. Therefore, thedifference in optical path length occurring in the red light beam 62 dueto the depth “h3” of the sawtooth grating is h3×(nr−1)/λ2=0.593, i.e.,about 0.6 times the wavelength λ2, as shown in FIG. 16C, and the phasemodulation amount becomes about 1.2π. Thus, the intensity of +1st-orderdiffracted light becomes highest at about 60% with respect to the redlight beam 62.

It is assumed that the shape of a grating in one period is a sawtoothcross-sectional shape with a depth “h3”, as shown in FIG. 16A. Since+1st-order diffracted light is strongest (in Embodiments 4 and 5,+2nd-order diffracted light is strongest even in the outer peripheralportion, which is different from the present embodiment) with respect tothe blue light beam 61, the grating period determining a diffractionangle is substantially p7, and a phase change becomes as shown in FIG.16B. With respect to the red light beam 62, +1st-order diffracted lightalso is strongest, and the grating period determining a diffractionangle also is substantially p7.

The outer peripheral portion 135B of the hologram 135 is designed sothat the blue light beam 61 is condensed through a base with a thicknessof about 0.1 mm. At this time, the red light beam 62 also is subjectedto +1st-order diffraction that is the same diffraction order as that forthe blue light beam 61, and the wavelength λ2 of the red light beam 62is longer than the wavelength λ1 of the blue light beam, so that adiffraction angle becomes large.

A blazing direction of the outer peripheral portion 135B of the hologram135 is designed so as to have a convex lens action, in the same way asin the inner circumferential portion 135C. At this time, since thediffraction angle is larger in the red light beam 62 than in the bluelight beam 61, the red light beam 62 is subjected to a strong convexlens action in the outer peripheral portion 135B of the hologram 135.This is completely different from, for example, the case where the redlight beam 62 is subjected to a convex lens action weaker than that forthe blue light beam 61 in the inner circumferential portion 134C of thehologram 134 according to Embodiment 4 or 5, or the case where the redlight beam 62 is subjected to a concave lens action in the innercircumferential portion 131C of the hologram 131 according toEmbodiment 1. Therefore, the red light beam 62 diffracted in the outerperipheral portion 135B is not condensed at the same place as that ofthe red light beam 62 passing through the inner circumferential portion135C.

Thus, a numerical aperture NAb when information is recorded/reproducedwith respect to the first optical disk 9 with the blue light beam 61 canbe made larger than a numerical aperture NAr when information isrecorded/reproduced with respect to the second optical disk 10 with thered light beam 62 (NAb>NAr).

Furthermore, the overall configuration of the optical head can becombined with the configuration additionally described in Embodiment 1.

Embodiment 7

Next, Embodiment 7 of the present invention will be described. Theoverall configuration of an optical head according to the presentembodiment is the same as that shown in FIG. 1 referred to in thedescription of Embodiment 1. In the present embodiment, theconfiguration of the hologram 13 shown in FIG. 1 is different from thoseof Embodiments 1 to 6.

In the present embodiment, as an intermediate embodiment betweenEmbodiments 4 and 5 described above, a depth “h4” of a sawtooth gratingin an inner circumferential portion of a hologram is set so as tosatisfy h2<h4<h1.

FIG. 17 is a graph showing a relationship between the depth “h” of thesawtooth grating formed in the inner circumferential portion 136C of thehologram 136 and the diffraction efficiency in the present embodiment.In FIG. 17, the horizontal axis represents how many times the differencein optical path length of the blue light beam 61 determined by the depth“h4” of the sawtooth grating becomes with respect to the wavelength λ1.The vertical axis represents a calculated value of the diffractionefficiency.

Setting the depth “h4” of the sawtooth grating so as to satisfy h2<h4<h1means selecting a value in a range that the horizontal axis (differencein optical path length/λ1) is larger than 1.7 and smaller than 2. Inparticular, (difference in optical path length/λ1) is selected to be1.88 (about 1.9) so that the diffraction efficiency (represented by abroken line) of +1st-order diffracted light of the red light beam 62 issubstantially equal to the diffraction efficiency (represented by asolid line) of +2nd-order diffracted light of the blue light beam 61.More specifically, the depth “h4” of the sawtooth grating is selected soas to satisfy h4×(nb−1)/λ1=1.88. Because of this, in terms ofcalculation, about 95% diffraction efficiency is obtained with respectto both +1st-order diffracted light of the red light beam 62 and+2nd-order diffracted light of the blue light beam 61. In both of thebeams, the loss of a light amount can be minimized.

Assuming that λ1 is 405 nm and the hologram material is BK7, “h4”satisfying the above condition becomes about 1.44 μm.

Embodiment 8

Next, Embodiment 8 of the present invention will be described. Theoverall configuration of an optical head according to the presentembodiment is substantially the same as that shown in FIG. 1 referred toin the description of Embodiment 1. In the present embodiment, theconfiguration of a complex objective lens composed of the hologram 13and the objective lens 14 shown in FIG. 1 is different from those ofEmbodiments 1 to 7.

FIG. 18 is a cross-sectional view showing a specific example of anobjective lens in the present embodiment. In FIG. 18, the objective lens147 of a refraction type in the present embodiment is composed of twocombined lenses: a first lens 1471 and a second lens 1472. The two-lenssystem has four refractive faces, so that its degree of freedom is high.The two-lens system can decrease the aberration occurring, for example,when the objective lens 147 is tilted with respect to the blue lightbeam 61 and the abaxial aberration. Thus, the two combined lenses canenhance the aberration characteristics of the objective lens. Inparticular, by setting the refractive surface outside of the first lens1471 (on the side away from the second lens 1472) to be an asphericsurface, the abaxial aberration can be decreased.

Furthermore, as described in Embodiment 1, by forming the hologram 137on the surface of the objective lens 147, the number of parts can bereduced. In particular, by forming the hologram 137 on the surfaceoutside of the first lens 1471 (on the side farthest from a condensedspot and the second lens 1472), the aberration occurring when theobjective lens 147 is tilted with respect to both of the red light beam62 and the blue light beam 61 can be decreased. As the hologram 137, anyof the hologram configurations of Embodiments 5 to 7 is used.

The above-mentioned sixth conventional example apparently is similar tothat of the present embodiment. However, the sixth conventional exampledoes not disclose that the refractive surface outside of the first lens1471 (on the side away from the second lens 1472) is set to be anaspheric surface, and hence, sufficient aberration characteristicscannot be obtained. In this respect, the sixth conventional example isdifferent from the present embodiment. Furthermore, the sixthconventional example also is different from the present embodiment inthat a red light beam is converted to strong dispersed light so as to beincident upon the hologram and the objective lens. Thus, in the sixthconventional example, a servo signal cannot be detected using a commonphotodetector with respect to a red light beam and a blue light beam.

Embodiment 9

FIG. 19 is a view showing a schematic configuration of an opticalinformation apparatus according to Embodiment 9 of the presentinvention. An optical information apparatus 67 of the present embodimentuses any of the optical heads of Embodiments 1 to 8.

In FIG. 19, a first optical disk 9 (or a second optical disk 10,hereinafter, this is the same) is placed on a turn table 82 and rotatedby a motor 64. An optical head 55 is moved roughly to a track on thefirst optical disk 9, at which desired information is present, by adriving device 51 of the optical head.

Furthermore, the optical head 55 sends a focus error signal and atracking error signal to an electric circuit 53 in accordance with thepositional relationship with the first optical disk 9. The electriccircuit 53 receives the signals and sends a signal for minutely movingan objective lens to the optical head 55. Because of this signal, theoptical head 55 reads/writes (records)/deletes information whileperforming focus control and tracking control with respect to the firstoptical disk 9.

The optical information apparatus 67 uses any of the optical heads ofEmbodiments 1 to 8. Therefore, a plurality of optical disks havingdifferent recording densities can be handled by a single optical head.

Embodiment 10

FIG. 20 is a view showing one exemplary configuration of a computeraccording to Embodiment 10 of the present invention. The computer 100according to the present embodiment includes the optical informationapparatus 67 according to Embodiment 9.

In FIG. 20, the computer 100 is composed of the optical informationapparatus 67, an input apparatus 101 such as a keyboard, a mouse, or atouch panel for inputting information, an arithmetic unit 102 such as acentral processing unit (CPU) for performing an arithmetic operationbased on information read from the optical information apparatus 67, andan output apparatus 103 such as a CRT display, a liquid crystal display,or a printer for displaying information on the result obtained by thearithmetic operation by the arithmetic unit 102. FIG. 18 illustrates thecase where a keyboard is used as the input apparatus 101, and a CRTdisplay is used as the output apparatus 103.

Embodiment 11

FIG. 21 is a schematic view showing one exemplary configuration of anoptical disk player according to Embodiment 11 of the present invention.The optical disk player 110 according to the present embodiment includesthe optical information apparatus 67 according to Embodiment 9.

In FIG. 21, the optical disk player 110 is composed of the opticalinformation apparatus 67, a decoder 111 for converting an informationsignal obtained from the optical information apparatus 67 to an imagesignal, and a liquid crystal monitor 112. In the present embodiment, theportable optical disk player 110 in which the liquid crystal monitor 112is formed integrally as a display has been illustrated and described.However, the display may be provided separately.

Embodiment 12

FIG. 22 is a view showing a schematic configuration of an automobile onwhich the car navigation system according to Embodiment 12 of thepresent invention is mounted. In FIG. 22, the car navigation system iscomposed of a GPS (Global Positioning System) 161, the optical diskplayer 110 according to Embodiment 11, and a display 163 for displayinga video signal from the optical disk player 110. Herein, the opticaldisk player 110 is not limited to this use, as long as it can reproduceinformation such as a video, a game, and a map from an optical disk.

In an automobile with such a car navigation system mounted thereon, avideo with a large capacity can be reproduced with a blue light beam,and detailed map data in a wide range can be handled. In addition, thereis a convenience that information recorded on an existing DVD also canbe used.

In the present embodiment, an automobile has been exemplified as avehicle. However, the present embodiment is not limited to anautomobile, and is applicable to other vehicles such as a train, anairplane, and a ship.

Embodiment 13

FIG. 23 is a schematic view showing one exemplary configuration of anoptical disk recorder according to Embodiment 13 of the presentinvention. The optical disk recorder 120 according to the presentembodiment includes the optical information apparatus 67 according toEmbodiment 9.

In FIG. 23, the optical disk recorder 120 is composed of the opticalinformation apparatus 67, an encoder 121 for converting an image signalto an information signal to be recorded on an optical disk, and adecoder 111 for converting the information signal obtained from theoptical information apparatus 67 to an image signal. An output apparatus103 such as a CRT display is connected to the optical disk recorder 120.Because of this, while an input image signal is converted to aninformation signal by the encoder 121 to be recorded on an optical disk,an information signal that has already been recorded on the optical diskis reproduced and converted to an image signal by the decoder 111,whereby the image signal thus obtained can be displayed on a CRT displaythat is the output apparatus 103.

Embodiment 14

FIG. 24 is a schematic view showing one exemplary configuration of anoptical disk server according to Embodiment 14 of the present invention.The optical disk server 150 according to the present embodiment includesthe optical information apparatus 67 according to Embodiment 9.

In FIG. 24, the optical disk server 150 is composed of the opticalinformation apparatus 67, a cable or wireless input/output terminal 151for taking an information signal to be recorded in the opticalinformation apparatus 67 from an outside and outputting an informationsignal read from the optical information apparatus 67 to the outside,and a changer 152 for loading/unloading a plurality of optical diskswith respect to the optical information apparatus 67. Furthermore, akeyboard as an input apparatus 101 and a CRT display as an outputapparatus 103 are connected to the optical disk server 150.

Because of the above, the optical disk server 150 can exchangeinformation with a network 153, i.e., a plurality of pieces of equipmentsuch as a computer, a telephone, and a TV tuner, and can be used as acommon information server with respect to these plurality of pieces ofequipment. Furthermore, by including the changer 152, a large amount ofinformation can be recorded/stored.

As described above, according to the present invention, a complexobjective lens having a high light use efficiency can be provided, whichrealizes compatible reproducing and recording between an optical diskwith a base thickness of 0.6 mm for recording and reproducing with a redlight beam having a wavelength λ2 (in general, about 660 nm) and anoptical disk with a base thickness of 0.1 mm for recording andreproducing with a blue light beam having a wavelength λ1 (in general,about 405 nm).

Furthermore, such a complex objective lens is used in an optical head,and such an optical head is mounted on an optical information apparatus,whereby a plurality of optical disks having different recordingdensities can be handled with a single optical head.

Furthermore, by including the above-mentioned optical informationapparatus in a computer, an optical disk player, an optical diskrecorder, an optical disk server, and a car navigation system,information can be recorded/reproduced stably with respect to differentkinds of optical disks, so that the present invention can be used in awide range of application.

Examples of different categories of optical information apparatusesinclude a computer, an optical disk player, a car navigation system, anoptical disk recorder, and an optical disk server.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof The embodiments disclosed inthis application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1. A complex objective lens comprising: a hologram; and a refractiontype lens, wherein said hologram has a grating having a steppedcross-sectional shape, a level difference of the stepped cross-sectionis an integral multiple of a unit level difference d1, the unit leveldifference d1 provides a difference in an optical path length of aboutone wavelength to a first light beam having a wavelength λ1 in a rangeof 390 nm to 415 nm, and one period of the grating is composed ofstepped heights in an order of zero time, twice, once, and three timesthe unit level difference d1 from an optical axis side of said hologramto an out peripheral side of said hologram.
 2. The complex objectivelens according to claim 1, wherein a ratio of widths of steps of thestepped cross-sectional shape of the grating is 2:3:3:2 corresponding tothe heights of the steps in the order of zero time, twice, once, andthree times the unit level difference d1.
 3. The complex objective lensaccording to claim 1, wherein the grating is formed only in an innercircumferential portion of the hologram.
 4. The complex objective lensaccording to claim 1, condensing 0th-order diffracted light of the firstlight beam through a base with a thickness t1 and condensing 1st-orderdiffracted light of a second light beam having a wavelength λ2 in arange of 630 nm to 680 nm through a base with a thickness t2 larger thanthe thickness t1.
 5. The complex objective lens according to claim 4,wherein in order to place a focal point position on an optical disk sideaway from the complex objective lens, the hologram is configured so asto have a larger function as a convex lens compared with a case ofcondensing the second light beam passing through an innercircumferential portion of the hologram through the base with thethickness t2, in a case of condensing the first light beam through thebase with the thickness t1, or the hologram is configured so as to havea smaller function as a convex lens compared with a case of condensingthe first light beam through the base with the thickness t1, in a caseof condensing the second light beam passing through the innercircumferential portion of the hologram through the base with thethickness t2.
 6. The complex objective lens according to claim 1,wherein the hologram and the refraction type lens are fixed integrally.7. The complex objective lens according to claim 1, wherein a refractivesurface of the refraction type lens on an opposite side of a condensedspot is an aspherical surface.
 8. The complex objective lens accordingto claim 7 wherein the hologram is formed integrally on the asphericalsurface of the refraction type lens.
 9. The complex objective lensaccording to claim 1, wherein the hologram is formed integrally on asurface of the refraction type lens.
 10. The complex objective lensaccording to claim 4, wherein, assuming that a numerical aperture atwhich the first light beam is condensed through the base with thethickness t1 is NAb, and a numerical aperture at which the second lightbeam is condensed through the base with the thickness t2 is NAr, NAb>NAris satisfied.
 11. A complex objective lens comprising: a hologram; and arefraction type lens, wherein said hologram includes a grating having astepped cross-sectional shape formed in at least an innercircumferential portion, a level difference of the steppedcross-sectional shape is an integral multiple of a unit level differenced2, the unit level difference d2 provides a difference in an opticalpath length of about 1.25 wavelengths to a first light beam having awavelength λ1 in a range of 390 nm to 415 nm, and one period of thegrating is composed of stepped heights in an order of zero time, once,twice, and three times the unit level difference d2 from an outerperipheral side of said hologram to an optical axis side of saidhologram.
 12. The complex objective lens according to claim 11, whereina ratio of steps of the stepped cross-sectional shape of the grating is1:1:1:1 corresponding to the heights in the order of zero time, once,twice, and three times the unit level difference d2.
 13. The complexobjective lens according to claim 11, wherein the hologram has a gratingwith a stepped cross-section formed in an outer peripheral portion, alevel difference of the stepped cross-section of the grating formed inthe outer peripheral portion is an integral multiple of a unit leveldifference d3, the unit level difference d3 gives a difference inoptical path length of about 0.25 wavelengths to the first light beam,and one period of the grating formed in the outer peripheral portion iscomposed of a step of heights in an order of 0 time, once, twice andthree times the unit level difference d3 from an outer peripheral sideto an optical axis side of the hologram.
 14. The complex objective lensaccording to claim 11, condensing +1st-order diffracted light of thefirst light beam through a base with a thickness t1 and condensing−1st-order diffracted light of a second light beam having a wavelengthλ2 in a range of 630 nm to 680 nm passing through a grating formed in aninner circumferential portion of the hologram through a base with athickness t2 larger than the thickness t1.
 15. The complex objectivelens according to claim 11, wherein the hologram is configured so as tohave a function as a convex lens in order to reduce a change in a focallength with respect to a change in the wavelength λ1 in a case ofcondensing the first light beam through a base with a thickness t1. 16.The complex objective lens according to claim 14, wherein in order toplace a focal point position on an optical disk side away from thecomplex objective lens, the hologram is configured so as to have alarger function as a convex lens compared with a case of condensing thesecond light beam passing through an inner circumferential portion ofthe hologram through the base with the thickness t2, in a case ofcondensing the first light beam through the base with the thickness t1,or the hologram is configured so as to have a smaller function as aconvex lens compared with a case of condensing the first light beamthrough the base with the thickness t1, in a case of condensing thesecond light beam passing through the inner circumferential portion ofthe hologram through the base with the thickness t2.
 17. The complexobjective lens according to claim 11, wherein the hologram and therefraction type lens are fixed integrally.
 18. The complex objectivelens according to claim 11, wherein a refractive surface of therefraction type lens on an opposite side of a condensed spot is anaspherical surface.
 19. The complex objective lens according to claim 18wherein the hologram is formed integrally on the aspherical surface ofthe refraction type lens.
 20. The complex objective lens according toclaim 11, wherein the hologram is formed integrally on a surface of therefraction type lens.
 21. The complex objective lens according to claim14, wherein, assuming that a numerical aperture at which the first lightbeam is condensed through the base with the thickness t1 is NAb, and anumerical aperture at which the second light beam is condensed throughthe base with the thickness t2 is NAr, NAb>NAr is satisfied.
 22. Anoptical head apparatus, comprising: a first laser light source foremitting a first light beam having a wavelength λ1 in a range of 390 nmto 415 nm; a second laser light source for emitting a second light beamhaving a wavelength λ2 in a range of 630 nm to 680 nm; a complexobjective lens for receiving the first light beam emitted from the firstlaser light source to condense 0th-order diffracted light of the firstlight beam onto a recording surface of a first optical disk through abase with a thickness t1, and receiving the second light beam emittedfrom the second laser light source to condense 1st-order diffractedlight of the second light beam onto a recording surface of a secondoptical disk through a base with a thickness t2 larger than thethickness t1; and a photodetector for receiving the first and secondlight beams reflected respectively from the recording surfaces of thefirst and second optical disks to output an electric signal inaccordance with light amounts of the first and second light beams,wherein the complex objective lens includes a hologram and a refractiontype lens, the hologram comprising a grating with a steppedcross-section, a level difference of the stepped cross-section is anintegral multiple of a unit level difference d1, the unit leveldifference d1 gives a difference in optical path length of about onewavelength to the first light beam, and one period of the grating iscomposed of a step of heights in an order of 0 time, twice, once, andthree times the unit level difference d1 from an outer peripheral sideto an optical axis side of the hologram.
 23. The optical head apparatusaccording to claim 22, comprising a collimator lens that collimates thefirst and second light beams respectively emitted from the first andsecond laser light sources, wherein when the second light beam iscondensed onto the recording surface of the second optical disk, thecollimator lens is placed closer to the second laser light source sideto convert the second light beam to divergent light so as to allow it tobe incident upon the complex objective lens, whereby a focal pointposition on the second optical disk side is placed away from the complexobjective lens.
 24. The optical head apparatus according to claim 22,wherein the first and second laser light sources are placed so that bothlighting points thereof have an image-forming relationship with respectto focal point positions of the complex objective lens on the first andsecond optical disk sides, and the photodetector is provided so as to beshared by the first and second light beams respectively reflected fromthe recording surfaces of the first and second optical disks andreceives the first and second light beams to detect a servo signal. 25.An optical head apparatus, comprising: a first laser light source foremitting a first light beam having a wavelength λ1 in a range of 390 nmto 415 nm; a second laser light source for emitting a second light beamhaving a wavelength λ2 in a range of 630 nm to 680 nm; a complexobjective lens for receiving the first light beam emitted from the firstlaser light source to condense +1st-order diffracted light of the firstlight beam onto a recording surface of a first optical disk through abase with a thickness t1, and receiving the second light beam emittedfrom the second laser light source to condense −1st-order diffractedlight of the second light beam onto a recording surface of a secondoptical disk through a base with a thickness t2 larger than thethickness t1; and a photodetector for receiving the first and secondlight beams reflected respectively from the recording surfaces of thefirst and second optical disks to output an electric signal inaccordance with light amounts of the first and second light beams,wherein the complex objective lens includes a hologram and a refractiontype lens, the hologram has a grating with a stepped cross-sectionformed in at least an inner circumferential portion, a level differenceof the stepped cross-section is an integral multiple of a unit leveldifference d2, the unit level difference d2 gives a difference inoptical path length of about 1.25 wavelengths to the first light beam,and one period of the grating is composed of a step of heights in anorder of 0 time, once, twice and three times the unit level differenced2 from an outer peripheral side to an optical axis side of thehologram.
 26. The optical head apparatus according to claim 25,comprising a collimator lens that collimates the first and second lightbeams respectively emitted from the first and second laser lightsources, wherein when the second light beam is condensed onto therecording surface of the second optical disk, the collimator lens isplaced closer to the second laser light source side to convert thesecond light beam to divergent light so as to allow it to be incidentupon the complex objective lens, whereby a focal point position on thesecond optical disk side is placed away from the complex objective lens.27. The optical head apparatus according to claim 25, wherein the firstand second laser light sources are placed so that both lighting pointsthereof have an image-forming relationship with respect to focal pointpositions of the complex objective lens on the first and second opticaldisk sides, and the photodetector is provided so as to be shared by thefirst and second light beams respectively reflected from the recordingsurfaces of the first and second optical disks and receives the firstand second light beams to detect a servo signal.
 28. An opticalinformation apparatus comprising: an optical head apparatus including: afirst laser light source for emitting a first light beam having awavelength λ1 in a range of 390 nm to 415 nm; a second laser lightsource for emitting a second light beam having a wavelength λ2 in arange of 630 nm to 680 nm; a complex objective lens for receiving thefirst light beam emitted from the first laser light source to condense0th-order diffracted light of the first light beam onto a recordingsurface of a first optical disk through a base with a thickness t1, andreceiving the second light beam emitted from the second laser lightsource to condense 1st-order diffracted light of the second light beamonto a recording surface of a second optical disk through a base with athickness t2 larger than the thickness t1; and a photodetector forreceiving the first and second light beams reflected respectively fromthe recording surfaces of the first and second optical disks to outputan electric signal in accordance with light amounts of the first andsecond light beams, wherein the complex objective lens includes ahologram and a refraction type lens, the hologram has a grating with astepped cross-section, a level difference of the stepped cross-sectionis an integral multiple of a unit level difference d1, the unit leveldifference d1 gives a difference in optical path length of about onewavelength to the first light beam, and one period of the grating iscomposed of a step of heights in an order of 0 time, twice, once, andthree times the unit level difference d1 from an outer peripheral sideto an optical axis side of the hologram; a motor for rotating the firstand second optical disks; and an electric circuit for receiving a signalobtained from the optical head apparatus and driving the motor, thecomplex objective lens, and the first and second laser light sourcesbased on the signal.
 29. The optical information apparatus according toclaim 28, wherein the optical head apparatus includes a collimator lensthat collimates the first and second light beams respectively emittedfrom the first and second laser light sources, and when the secondoptical disk having a base with a thickness t2 of 0.6 mm is mounted, theoptical information apparatus moves the collimator lens to the secondlaser light source side.
 30. An optical information apparatuscomprising: an optical head apparatus including: a first laser lightsource for emitting a first light beam having a wavelength λ1 in a rangeof 390 nm to 415 nm; a second laser light source for emitting a secondlight beam having a wavelength λ2 in a range of 630 nm to 680 nm; acomplex objective lens for receiving the first light beam emitted fromthe first laser light source to condense +1st-order diffracted light ofthe first light beam onto a recording surface of a first optical diskthrough a base with a thickness t1, and receiving the second light beamemitted from the second laser light source to condense −1st-orderdiffracted light of the second light beam onto a recording surface of asecond optical disk through a base with a thickness t2 larger than thethickness t1; and a photodetector for receiving the first and secondlight beams reflected respectively from the recording surfaces of thefirst and second optical disks to output an electric signal inaccordance with light amounts of the first and second light beams,wherein the complex objective lens includes a hologram and a refractiontype lens, the hologram has a grating with a stepped cross-sectionformed in at least an inner circumferential portion, a level differenceof the stepped cross-section is an integral multiple of a unit leveldifference d2, the unit level difference d2 gives a difference inoptical path length of about 1.25 wavelengths to the first light beam,and one period of the grating is composed of a step of heights in anorder of 0 time, once, twice, and three times the unit level differenced2 from an outer peripheral side to an optical axis side of thehologram; a motor for rotating the first and second optical disks; andan electric circuit for receiving a signal obtained from the opticalhead apparatus and driving the motor, the complex objective lens, andthe first and second laser light sources based on the signal.
 31. Theoptical information apparatus according to claim 30, wherein the opticalhead apparatus includes a collimator lens that collimates the first andsecond light beams respectively emitted from the first and second laserlight sources, and when the second optical disk having a base with athickness t2 of 0.6 mm is mounted, the optical information apparatusmoves the collimator lens to the second laser light source side.
 32. Theoptical information apparatus according to claim 28, which is in theform of one selected from the group consisting of a computer, an opticaldisk player, a vehicle navigation system, an optical disk recorder andan optical disk server.
 33. The optical information apparatus accordingto claim 30, which is in the form of one selected from the groupconsisting of a computer, an optical disk player, a vehicle navigationsystem, an optical disk recorder and an optical disk server.